These puzzles give rules for a number machine, lights or codes, solved by writing down each step. Children in Year 3 now sit the OC test in Year 4, on 11–12 May 2027.
They are puzzles about a system that follows rules. Something goes in, the rules act on it, and something comes out. There are three kinds your child will meet.
Number machines: if the number is even, halve it; if it's odd, add 5. Start at 18 and run it three times: 18, 9, 14, 7.
Lights and buttons: each button changes certain lights. A light that is off turns on, and a light that is on turns off.
Secret codes: compare a word with its code to find the rule, such as moving every letter two places on in the alphabet, then use the rule on a new word.
How do these puzzles show up in the OC Thinking Skills test?
Your child reads the rules, then works out the result after a few steps, or decodes a new word. 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 these puzzles
30 questions in 30 minutes
Steady steps on paper are faster than redoing it
4 options each
Wrong options are often the answer one step early or late
There are no marks lost for incorrect answers
If time runs short, check an option against the rules
The Department doesn't publish a fixed list of question types. Systems and switches is how our lessons group these questions.
Four steps. The secret is a small table on working paper, with one line for each step.
Write the rules short
Even: halve; odd: add 5; red: lights 1, 2, 3. Short notes are easier to follow than long sentences.
Write the starting state
Which number goes in? Which lights are on at the start? Write it as the first line.
One step, one line
Apply one rule at a time and write the new state on a new line. Count the steps so you stop at the right one.
For codes, test the rule on every example
Compare each letter with its code. Check the rule works for both examples before using it on the new word.
What do OC code and switch puzzles look like?
Three questions from easier to harder: a number machine, a row of lights and a secret code.
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
A number machine follows these rules:
If the number that goes in is even, the machine halves it.
If the number that goes in is odd, the machine adds 5.
The number that comes out goes straight back into the machine.
The number 18 goes into the machine. What number comes out after the machine has worked 3 times?
AA.
12
BB.
9
CC.
7
DD.
14
Answer: C
Answer: C (7)
First time: 18 is even, so it is halved: 9.
Second time: 9 is odd, so 5 is added: 14.
Third time: 14 is even, so it is halved: 7.
Why the other options are wrong
A (12): 12 comes from adding 5 to 7, which would be a 4th time through the machine.
B (9): 9 is what comes out after only 1 time through the machine.
D (14): 14 is what comes out after only 2 times through the machine.
Worked explanation 2
Five lights in a row are numbered 1 to 5. At the start they are all off. Each button changes some of the lights: a light that is off turns on, and a light that is on turns off.
The red button changes lights 1, 2 and 3.
The blue button changes lights 2 and 4.
The green button changes lights 3, 4 and 5.
Tayla presses the red button, then the blue button, then the green button. Which lights are on at the end?
AA.
Lights 1 and 5
BB.
All five lights
CC.
Lights 1, 3 and 4
DD.
Lights 3, 4 and 5
Answer: A
Answer: A (Lights 1 and 5)
Keep track of the lights after each press.
Red: lights 1, 2 and 3 turn on. On: 1, 2, 3.
Blue: light 2 turns off and light 4 turns on. On: 1, 3, 4.
Green: lights 3 and 4 turn off and light 5 turns on. On: 1 and 5.
Why the other options are wrong
B (All five lights): This treats every press as turning lights on, but a light that is already on turns off.
C (Lights 1, 3 and 4): These are the lights that are on after the blue button; the green button still has to be pressed.
D (Lights 3, 4 and 5): These are only the lights the green button changes; the earlier presses matter too.
Worked explanation 3
In a secret code, DOG is written as IQF, and SUN is written as PWU.
How is MAP written in the same code?
AA.
OCR
BB.
PAM
CC.
QBN
DD.
RCO
Answer: D
Answer: D (RCO)
Read IQF backwards: F, Q, I. Each of these letters is two places after the matching letter of DOG in the alphabet: D to F, O to Q, G to I.
SUN works the same way: PWU backwards is U, W, P, and S to U, U to W, N to P are each two places on.
So the code moves every letter two places on in the alphabet, then writes the word backwards.
MAP: M to O, A to C, P to R gives OCR. Written backwards, that is RCO.
Why the other options are wrong
A (OCR): Each letter has moved two places on, but the word has not been written backwards.
B (PAM): This writes MAP backwards but does not change any of the letters.
C (QBN): This writes the word backwards but moves each letter only one place on; the examples move two places.
Which mistakes cost marks in these puzzles?
Most wrong options are a correct state from the wrong step. Each has a quick check your child can run before choosing.
×
Stopping one step early or late: number each line, and stop when the count matches the question.
×
Treating every press as turning lights on: a light that is already on turns off, so check its last state.
×
Finding half the code rule: if the letters move but the answer still doesn't match, look for a second change, such as writing the word backwards.
What does the Systems, Switches & State Changes lesson cover?
Systems, Switches & State Changes (lesson preview)
A short preview from the SelectiveReady lesson “Systems, Switches & State Changes”, on switch and state puzzles for the NSW Selective test.
Published:
Read the video transcript
Okay, step one, we press button A. Now, what did our rules say? A toggles lights one and three. They were both off, so boom, they both turn on. Our new setup is on, off, on, off. All right, next we hit button B. This one affects lights two, three, and four. So lights two and four, they were off, now they're on. But wait, what about light three? It was already on. So toggling it means it goes back off. See how that works? Our new state is on, on, off, on. And now for the grand finale. We press button A one more time. It hits lights one and three. Light one was on, so now it's off. Light three was off, so now it's on. And remember, button A doesn't touch lights two and four, so they just stay put. And just like that, we're done. After that whole A-B-A sequence, what's our final answer? Light one is off. Lights two, three, and four are all on. Puzzle, solved.
The preview, with English and Chinese captions, presses buttons one at a time and tracks which lights are on after each press. The full lesson in the SelectiveReady app adds number machines, codes and the common traps. A free account opens one lesson, How and Why Authors Write; the other lessons need a paid plan.
How can my child practise code and switch puzzles for free?
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 these puzzles.
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, write secret messages to each other with a simple letter-shift code, or invent a number machine and guess each other's rule.
