Selective thinking skills: necessary assumptions practice

Quick Answer

A necessary assumption is an unstated idea that must be true for an argument's reason to support its conclusion. To test an option, make it false and see whether the argument falls over. This page from Braintree Coaching Australia has eight original arguments for Year 5–6 children preparing for NSW Selective thinking skills, with the negation test written out for every answer. It is free and needs no sign-up.

Who is this assumption practice for?

This set suits a Year 5 or Year 6 child who can already pick out the conclusion of a short argument and now needs to work out what the argument takes for granted. It is skill practice. We do not claim these items match questions on the real test.

The NSW Department of Education lists four sections in the Selective High School Placement Test: Reading, Mathematical Reasoning, Thinking Skills and Writing. Its page gives the Thinking Skills section as 40 multiple-choice questions in 40 minutes. See the department's Selective practice tests for the real format, and our guide to the Thinking Skills format for how the section works.

How does the negation test work? A worked model

Argument: "Our netball team has won every game since we changed to the blue uniform. The blue uniform must be helping us win."

Candidate assumption: "Nothing else important changed at the same time as the uniform."

1. Find the conclusion. The blue uniform is helping the team win.
2. Find the reason. The team has won every game since the change.
3. Make the candidate false. Suppose something else important did change at the same time. Say a strong new shooter joined that week.
4. Re-read the argument. Now the wins could be because of the new player. The reason no longer points to the uniform.
5. Decide. Making the candidate false broke the argument, so the argument needs it. It is a necessary assumption.

Try the same steps on "Blue is the team's favourite colour". Make it false: blue is not their favourite. The argument is untouched, because wins after the change still seem to point to the uniform. So that one is not needed.

The questions

For each argument, choose the option the argument needs to be true. Use the negation test before you look at the answers.

Question 1. "Mia's school banned phones at lunchtime this term. Since the ban, students have borrowed more library books. So the ban has led students to read more."

• A. Phones make it harder for children to learn.
• B. Students read at least some of the extra books they borrow.
• C. Every student has borrowed a book this term.
• D. The library bought many new books this term.

Question 2. "The footbridge near the school is crowded at 8:30 am on school days. So the council should build a second footbridge beside it."

• A. Some of the people who crowd the first bridge would use the second one.
• B. The first bridge is crowded all day.
• C. A second footbridge would be cheap to build.
• D. No families drive their children to school.

Question 3. "Tom scored higher on this week's spelling test than on last week's. He must have studied harder this week."

• A. Tom studies for every spelling test.
• B. Spelling is Tom's best subject.
• C. This week's test was not much easier than last week's.
• D. Both tests were held on a Friday.

Question 4. "The tomatoes in the school garden were planted in the sunny bed and grew well. So the beans will grow well if we plant them in the sunny bed too."

• A. Beans respond to a sunny position in a similar way to tomatoes.
• B. Tomatoes grow faster than beans.
• C. The shady garden bed is already full.
• D. Beans need less water than tomatoes.

Question 5. "The swimming club has twice as many members as last year. So the club needs to hire more coaches."

• A. Good coaches are easy to find.
• B. The club's membership will double again next year.
• C. The club has more than one pool.
• D. The new members take part in coached sessions.

Question 6. "Monday is forecast to be wet and Friday is forecast to be dry. So our class excursion to the wetlands should move from Monday to Friday."

• A. Weather forecasts are always correct.
• B. Wet weather would make the excursion worse.
• C. No student in the class has ever been away on a Friday.
• D. The excursion costs less on a Friday.

Question 7. "A survey of 200 people leaving the science museum found that 90% enjoyed their visit. So most people in the town would enjoy the museum."

• A. Two hundred is the largest number of people who have visited in one day.
• B. Entry to the museum is free.
• C. The people surveyed are similar to the rest of the town in what they enjoy.
• D. The other 10% of people surveyed dislike science.

Question 8. "Since the new timetable began, school buses arrive five minutes earlier. So fewer students will be late to class."

• A. Some students were late because their bus got to school late.
• B. Every student at the school catches a bus.
• C. The buses are never delayed.
• D. Students prefer the new timetable.

Answers and worked explanations

Question 1: B. Negate it: students read none of the extra books they borrow. Then more borrowing tells us nothing about more reading, and the conclusion fails. A is a general opinion about phones that the argument never relies on. C is far stronger than needed; borrowing can rise without every student taking part. D would weaken the argument by giving another reason for the extra borrowing, so it cannot be something the argument depends on.

Question 2: A. Negate it: nobody from the crowd would use the second bridge. Then building it leaves the crowding unchanged, and the reason gives no support for the conclusion. B is not needed, because crowding at one busy time is the whole reason given. C is about cost, which the argument never mentions. D is too strong and beside the point.

Question 3: C. Negate it: this week's test was much easier. Then a higher score is explained by the easier test, and we cannot conclude Tom studied harder. A does not help: if he always studies, that says nothing about studying harder this week. B is irrelevant to a change between two weeks. D is a detail about the day that has nothing to do with how hard Tom studied.

Question 4: A. Negate it: beans do not respond to a sunny position the way tomatoes do. Then the tomatoes' success tells us nothing about beans. B compares growth speed, which is not the point. C explains why someone might choose the sunny bed, not why the beans would grow well there. D is about water, which the argument does not use.

Question 5: D. Negate it: the new members do not take part in coached sessions (perhaps they only swim laps on their own). Then twice the members does not mean more coaching is needed. A is about whether hiring is easy, not whether it is needed. B is a prediction about next year. C is about pools, not coaches.

Question 6: B. Negate it: wet weather would not make the excursion worse. Then there is no reason to move it away from the wet day. A is the trap. The argument needs forecasts to be reasonably reliable, not always correct. Negate A to "forecasts are sometimes wrong" and the argument still stands. C and D bring in ideas the argument does not depend on.

Question 7: C. Negate it: the people surveyed are not like the rest of the town. People who choose to visit a science museum may already like science more than most. Then their enjoyment does not tell us about everyone else. A and B are facts about the museum that do not connect the survey to the town. D guesses at why 10% did not enjoy it, which the conclusion does not need.

Question 8: A. Negate it: no student was late because their bus got to school late. Then earlier buses change nothing, and the conclusion fails. B is too strong; the argument works if only some students catch buses. C is also too strong; buses can be delayed occasionally and still arrive earlier on most days. D is about what students like, not about lateness.

One more point on Question 8: an argument can rest on more than one assumption. This one also needs class to start at the same time as before. That was not among the options, and the question only asks which listed option is needed.

What mistake patterns should we look for?

Pattern	Seen in	What to practise next
Chose an option that is too strong (all, every, always, never)	Questions 1C, 6A, 8B, 8C	Ask "would the argument survive if this were only mostly true?"
Chose something true-sounding but unconnected to the reason	Questions 1A, 3B, 7B	Underline the reason and the conclusion, then check the option touches both
Chose an option that weakens the argument	Question 1D	Decide first whether the option helps or hurts the argument
Skipped the negation step	Any	Write the negated sentence out in full for three questions

Can my child try the skill on a new argument?

Read this argument and write down, in your own words, one thing it needs to be true. There are no options this time.

"Our street had three bike accidents last year. This year the council painted a bike lane, and there has been only one accident. So the bike lane has made the street safer."

Self-check: a good answer is something like "about as many people rode bikes on the street this year as last year" or "nothing else changed that would reduce accidents". Negate either one and the fall in accidents no longer points to the bike lane.

What should we do after this set?

• If most errors were "too strong" choices, redo Questions 6 and 8 tomorrow and explain aloud why each strong option is not needed.
• For mixed Selective practice and a worked sample across sections, open the NSW Selective practice tests and resources guide.
• The free mock tests need a free classroom account. Use one when your child is ready for a timed sitting.
• If you want a structured set of full Selective tests after that, see what the Selective course includes and check it matches your child's entry year before buying.

Sources and review

Official source (checked 2026-10-05): NSW Department of Education, Selective High School practice tests: four sections; Thinking Skills is 40 multiple-choice questions in 40 minutes.

Where the questions came from: all eight arguments, the worked model and the transfer task are original Braintree Coaching Australia material written for this page. None is taken from an official or third-party paper.

Author: James Rush, content writer, Braintree Coaching Australia.

Answer check: every answer was solved a second time, independently of the author, before publication. A named teacher review has not yet been recorded for this set. If you find an error, please tell us.

Last updated: 2026-10-05.