Maths question that I really need help on for explaining.

Question

By rounding each value to one significant figure estimate the cost of 3.9kg of apples at 87p per kg

Answer 1

3.9kg will now be 4 kg since it's really close to 4 and that way it will be one significant figure

87p/kg will now be 90p/kg since it's closer to 90 than 80 and that way it will be one significant figure

(4kg)*(90p/kg)=360p