These original secondary-level examples show how a small error can change an answer. The speed example suits work on motion at KS3; the chemistry and biology examples illustrate skills in AQA GCSE Combined Science: Trilogy (8464). They are practice illustrations, not past-paper questions.
Physics: keep the time unit with the speed
A cyclist travels 150 metres in 2 minutes. A learner writes “150 ÷ 2 = 75 m/s”. The division is valid, but its unit is metres per minute. To give average speed in metres per second, first convert 2 minutes to 120 seconds. Then 150 ÷ 120 = 1.25 m/s.
A better response is: “The cyclist’s average speed is 1.25 m/s because the total distance is divided by the time in seconds.” Multiplying 1.25 by 120 gives the original 150 metres, which checks the calculation. This tests both the maths of conversion and the physical meaning of average speed.
Chemistry: scale the amount to the solution volume
A solution has a concentration of 12 g/dm³: each cubic decimetre, or litre, of solution contains 12 grams of dissolved substance. How much is in 250 cm³ of that solution? Multiplying 12 by 250 to get 3,000 grams mixes the two volume units.
Since 1 dm³ = 1,000 cm³, the sample is 0.250 dm³, a quarter of a litre. The mass of dissolved substance is 12 × 0.250 = 3.0 g. “A quarter of the volume contains a quarter of the mass” explains why the answer is reasonable. The volume here is the solution’s volume.
Biology: use the starting mass for percentage change
Suppose a piece of plant tissue has a mass of 2.0 g before an investigation and 2.2 g afterwards. Its gain is 0.2 g. Divide by the starting mass, not the final mass: 0.2 ÷ 2.0 × 100 = 10%.
A careful response is: “The tissue’s mass increased by 10%.” That describes the data. Explaining the increase requires information about the investigation, including the conditions and how the measurements were taken; these two numbers alone do not show that osmosis caused it. Here the tutor needs to check both the percentage method and the scientific explanation.
