An assumption is a condition your answer relies on that the question did not state. Find it by asking: what must be true for my answer to be correct?
The what-must-be-true question
A worked example: a bus travels 60 km in 1.5 hours, and a student says that a 100 km trip will take 2.5 hours.
The working is: speed = 60 ÷ 1.5 = 40 km/h, then time = 100 ÷ 40 = 2.5 hours. The answer depends on the bus keeping 40 km/h for the whole journey, through traffic and stops. That is the assumption.
Stating the condition in your answer
A bare answer “2.5 hours” reads as a fact. A careful answer reads “2.5 hours, assuming the bus keeps a constant speed of 40 km/h”.
The second version is not longer by much, and it tells the reader what the number depends on. In Science and modelling questions, that sentence is often the mark.
A second example from a tap
A tap fills a 10 litre bucket in 4 minutes. A student says a 50 litre tank fills in 50 ÷ 10 × 4 = 20 minutes.
What must be true? The flow rate stays constant, the tank starts empty, and the tank is not leaking. If the tank is already half full, the time is about 10 minutes, so the assumption matters a great deal.
Where assumptions hide
- Physics: friction and air resistance are ignored, or acceleration is constant.
- Chemistry: the reaction goes to completion, or the gas is at room conditions.
- Mathematics: growth continues at the same rate, or values are exact rather than rounded.
- Everyday problems: prices, speeds and rates stay the same over time.
Assumptions are not mistakes. They are simplifications that make a problem solvable, provided you know they are there.
The oversight to avoid
Students sometimes treat a model answer as a prediction of reality. Saying “the bus will arrive at 2.5 hours” is stronger than the working supports. Replace it with “the trip takes 2.5 hours at constant speed”.
Try it yourself
A printer prints 12 pages in 30 seconds. A student says 120 pages take 5 minutes. State one assumption and check the arithmetic.
Answer
Rate = 12 ÷ 30 = 0.4 pages per second. 120 ÷ 0.4 = 300 seconds = 5 minutes, so the arithmetic is right.
The assumption is that the printer keeps the same speed throughout, with no paper refills, warm-up or jams. A large job could also slow down if the paper tray empties.
Next steps
Assumptions and evidence go together, so revisit checking whether evidence supports a claim. The hub lists the other habits, and SPM Science tuition is available for one-to-one practice.