A deduction question gives clues and asks what must be true, not just what could be. Children in Year 3 now sit the OC test in Year 4, on 11–12 May 2027.
Deduction means using clues to reach an answer that must be true. Every fish in a tank is orange or white, and orange fish never have stripes. Dot has stripes, so Dot must be white: there is no other way.
The clues are the only truth. Real-life knowledge stays outside, and so do guesses. An answer that might be true is not good enough; the clues have to lock it in.
How do deduction questions show up in the OC Thinking Skills test?
Your child reads a few short clues about a class, a pet or a game, then picks the statement that must be true, or the person the clues point to. The Department says Thinking Skills checks critical thinking and problem solving, with no previous knowledge needed, and the section has a range of question types, shown in the practice tests.
The OC Thinking Skills test
What it means for deduction
30 questions in 30 minutes
Write the clues down briefly before testing options
4 options each
Ask of each one: must this be true?
There are no marks lost for incorrect answers
Choose the option the clues force, then move on
The Department doesn't publish a fixed list of question types. Deduction is how our lessons group these questions.
Four steps, the way a detective works. The last one matters most: an option that could be false is out.
List the clues exactly
Write each clue as it is given. Words like every, none, only and more than half matter most.
Join two clues
Put clues together to find a new fact. Dot has stripes and orange fish have none, so Dot isn't orange.
Try each possibility
When there are a few suspects, pretend each one did it and check the clues. Keep only the one that fits.
Ask must, not might
For each option, look for any way it could be false. If there is one, cross it out.
Numbers can be clues too. With 24 children voting, more than half means 13 votes or more, because half is 12.
What do OC deduction questions look like?
Three questions from easier to harder: a fish tank, a class vote and a puzzle where just one of three children tells the truth.
Each one shows the answer and the working, so read them with your child. The explanation says why each wrong option is wrong: ask your child which one looked right at first, because that's the habit to fix.
Worked explanation 1
In Mr Kim's fish tank, every fish is either orange or white. None of the orange fish has stripes.
Dot is a fish in Mr Kim's tank, and Dot has stripes. Which one of these must be true?
AA.
Dot is a white fish.
BB.
Dot is an orange fish.
CC.
Every white fish in the tank has stripes.
DD.
Dot is the only fish in the tank with stripes.
Answer: A
Answer: A (Dot is a white fish.)
Every fish in the tank is orange or white.
Orange fish never have stripes. Dot has stripes, so Dot cannot be orange.
The only colour left is white, so Dot is a white fish.
Why the other options are wrong
B (Dot is an orange fish.): Orange fish have no stripes, so a fish with stripes cannot be orange.
C: We only know about Dot, so some white fish might have no stripes at all.
D: Other white fish might have stripes too; nothing says Dot is the only one.
Worked explanation 2
There are 24 children in Ms Nguyen's class. Each child voted once for a class mascot: a koala, an emu or a platypus. The koala got more than half of the votes.
Which one of these must be true?
AA.
The platypus got at least 1 vote.
BB.
The emu got fewer than 12 votes.
CC.
The koala got exactly 13 votes.
DD.
The emu got more votes than the platypus.
Answer: B
Answer: B (The emu got fewer than 12 votes.)
Half of 24 is 12, so "more than half" means the koala got 13 votes or more.
That leaves at most 24 − 13 = 11 votes for the emu and the platypus together.
So the emu got 11 votes or fewer, which is always fewer than 12.
Why the other options are wrong
A (The platypus got at least 1 vote.): All the other votes could have gone to the emu, so the platypus might have no votes.
C (The koala got exactly 13 votes.): 13 is the smallest number of votes the koala could have, but it could also have 14 or more.
D: We are not told how the emu and platypus votes were split, so either one could have more.
Worked explanation 3
Someone left the back gate open. It was Zara, Kai or Noah.
Zara says: "Kai did it."
Kai says: "I did not do it."
Noah says: "I did not do it."
Only one of the three children is telling the truth.
Who left the back gate open?
AA.
Kai
BB.
It cannot be worked out.
CC.
Noah
DD.
Zara
Answer: C
Answer: C (Noah)
Try each child in turn and count how many sentences are true.
If Zara did it: Kai's sentence and Noah's sentence are both true. That is two true sentences, so it was not Zara.
If Kai did it: Zara's sentence and Noah's sentence are both true. Two again, so it was not Kai.
If Noah did it: Zara's sentence is false, Kai's is true and Noah's is false. Exactly one is true, which matches.
So Noah left the gate open.
Why the other options are wrong
A (Kai): Zara's sentence points at Kai, but if Kai did it, Zara and Noah would both be telling the truth.
B (It cannot be worked out.): Testing each child shows that exactly one of them fits, so it can be worked out.
D (Zara): If Zara did it, both Kai and Noah would be telling the truth, which is two children, not one.
Which deduction mistakes cost marks?
Most wrong options are possible but not certain. These are the three to watch. Each has a quick check your child can run before choosing.
×
Choosing an option that could be true: the answer must be true in every case, so look for a way each option fails.
×
Treating the smallest possible number as the exact number: more than half of 24 could be 13 or any number above it.
×
Believing the person who points a finger: in truth-teller puzzles, try each suspect in turn and count how many sentences are true.
What does the Thinking Like a Detective lesson cover?
Thinking Like a Detective (lesson preview)
A short preview from the SelectiveReady lesson “Thinking Like a Detective”, on deductive reasoning for the NSW Selective test.
Published:
Read the video transcript
Let's start digging. So, our investigation is going to focus on our two persons of interest, Tom and Sarah. For each of them, we're going to apply our all rule and check every possible conclusion against that tough, must-be-true standard. Remember, no assumptions. Let's break down the deductions. Okay, first up, Tom. The clues say he is in the chess club, and the clues also say that all students in the chess club wear glasses. So does Tom have to wear glasses? Yes, it must be true. That's a solid, confirmed deduction. Now, what about Sarah? We know she wears glasses. Does that mean she must be in the chess club? Ah, hold on. This is that invalid reversal trap we talked about. Just because all chess club members wear glasses doesn't mean everyone who wears glasses is in the chess club. So could she be in the club? Maybe, but is it certain? Absolutely not. And case closed. See, by sticking to the code, we found the right answer.
The preview, with English and Chinese captions, checks two children against a club rule and asks of each conclusion: must it be true?
The full lesson in the SelectiveReady app adds clue words like all, some and none, circles to join clues, and the common traps. A free account opens one lesson, How and Why Authors Write; the other lessons need a paid plan.
Start with Practice test 1, which has Thinking Skills at its real length and time. It's one full practice test with results, with a free account. Every answer is explained, and the results by skill show whether your child loses marks on deduction.
After that, the app has drills by section and topic; a free account drills the free test's questions, and a paid plan opens the rest. At home, play must, might or can't: give two facts and a sentence, and your child says whether it must be true, might be true or can't be true.
