We know area of A – area of B = 5 sq cm
Looking at Area of Big Triangle QRT – Area of rectangle PQRS
—> means trapezium part of QRT is taken away by the trapezium part of PQRS
—> left with area of A – area of B = 5
so 1/2 x 10 x 9 – PQ x 10 = 5
45-5 = PQ x 10
PQ = 4 cm#
updated 19/5/17: attached picture, if it helps in explaining the workings above. I learnt this method (of “markings” and “cancelling out” in diagram) about 2-3 years ago from school’s Maths workshop for parents and from there, it built some confidence in the PSLE curriculum for me and understand that sometimes, kids may be taught simpler methods in school nowadays that our old school didn’t.