Why? Even if the answer is wrong, some marks could still be awarded to the workings and model drawings. Take it as consolation.
This tip is very applicable to some very smart children who can do complex workings in their mind. Leave the mental calculations to later in their years; for now, encourage your child to pen down all workings and model drawings.
Have your child encountered this situation before?
Question extracted from: "P5 Must Know Maths Problem Sums & Model Methods"
Experiment #7 - Forces (Frictional Force)
Experiment #6 - Forces (Frictional Force)