For parents · Primary & PSLE math
Stop tracking the water level: the one habit that cracks tank questions
The tank question is the one so many children dread at the end of Paper 2. There is one habit that turns it from a maze into a sum, and it is small enough to teach in an afternoon.
Mrs Eileen Toh
Founder & Curriculum Architect · ex-MOE · 5 min read · Updated 22 Jun 2026
If your child has ever come home from a paper and said "the tank question got me again," you are in very good company. It is one of the questions that turns confident children anxious, and the reason is almost always the same: they are trying to track the wrong thing.
So here is the one habit I most want to hand you, and it is genuinely the whole secret. A PSLE tank question is a volume question, not a level question. The moment a child stops watching the water level and starts working in volume, the question that looked like a maze becomes an ordinary sum.
Why the level lies, and volume tells the truth
Water levels rise and fall, and that movement is exactly what catches the eye and the worry. Worse, two tanks with different bases have levels that simply cannot be compared: the same amount of water sits high in a narrow tank and low in a wide one. A level is a height, and heights from different tanks are not the same currency.
Volume is. Cubic centimetres are cubic centimetres, and litres are litres, in any tank of any shape. So the reliable habit has two parts. First, work in volume, in cubic centimetres or litres, never in the level. Second, count each tap's minutes separately, because taps that start at different times have run for different lengths of time, and that difference is usually where the marks are hiding.
A worked example, in plain words
Let me take you through a real one, the 2021 Paper 2 Question 17, entirely in words so you can see the habit in action. A tank measures 55 cm by 30 cm by 20 cm and holds 33 litres. Tap A pours water in at 4.2 litres a minute, starting at 2.00 pm. Tap B starts draining the tank five minutes later. At 2.15 pm the tank is exactly half full, which is 16.5 litres.
Now we work in litres, and we count each tap separately. By 2.15 pm, tap A has run for 15 minutes, so it has poured in 15 times 4.2, which is 63 litres. Tap B, however, started five minutes later, so it has run for only 10 minutes. The tank holds 16.5 litres at 2.15 pm, so the amount tap B drained out must be 63 minus 16.5, which is 46.5 litres in 10 minutes. That gives tap B a draining rate of 4.65 litres a minute.
From 2.15 pm to 2.30 pm, both taps run for the same 15 minutes. Tap A adds another 63 litres, and tap B drains 15 times 4.65, which is 69.75 litres. Starting from 16.5 litres, the tank now holds 16.5 plus 63 minus 69.75, which is 9.75 litres. So the tank is 9.75 out of 33 full, and that fraction simplifies to 13/44 full.
The whole trick is that five-minute head start. Tap A ran for 15 minutes while tap B ran for only 10, and a child who counts both taps from the same moment will be wrong by exactly that gap. Counting each tap's minutes separately is not a flourish; it is the question.
Paper 2s with a tank
a tank that fills, in every Paper 2 for fourteen years
not level
the one habit that turns the maze into a sum
The 2021 answer
how full the tank is, once you work in litres
Why this habit is worth building
A tank that fills has appeared in every PSLE Paper 2 for fourteen years running, fourteen of fourteen in the papers we counted. That is not a prediction about the next paper, and I would be wary of anyone who turned it into one. It is simply a statement about what has happened, and it means this is a structure worth teaching to mastery rather than hoping to avoid.
The good news for parents is how portable the habit is. "Work in volume, not level. Count each tap's minutes separately." Those two sentences carry across one tank or two, taps that fill or taps that drain, water poured between containers. The story changes; the habit does not. If you would like to go deeper, our type guide on PSLE volume and rate questions works through more of these step by step, and you can see honest analysis of the hardest recurring questions at our hardest-questions hub.
An honest word on practice
One worked example does not build a habit; a handful of similar questions, done with the same two rules each time, does. So I would not expect a single read of this post to fix the tank question for good. What I would expect is that the next time your child meets one, they reach for volume instead of level, and they pause to ask how long each tap has actually been running. That small change of where their attention goes is, in my experience, the most dependable thing you can do for this question.