Mechanics - Maths A-Level

http://www.ocr.org.uk/Images/78549-question-paper-unit-4728-mechanics-1.pdf

How do I go about doing 4) iii)

The greatest magnitude of the resultant force is achieved when all forces act in the same direction. Hence the greatest magnitude is $8+15+20=43N$.



For 'the least possible magnitude of resultant force' part. Notice that

$\sqrt{8^2+15^2} = 17$

Hence the minimum resultant force is achieved when the combination of the 8N force and 15N force acts in the opposite direction on the same straight line as the 20N force. This means the minimum resultant force is equal to $20-17=3N$.

Ah i see thanks, first bit makes sense but in the mark scheme it says the least magnitude is 0?