The center of gravity is on QR, and the relationship between b and n can be obtained.

Angle RPQ90, it is faster to do it by vector vertical method.

Triangle CRQ is similar to BPQ. The ratio of CQ and QB is equal to the ratio of CR and BP.

Three equations can solve the unknown. Because I called from my mobile phone, I omitted some details. )

However, if you encounter this problem in the examination room, it will save time to use the substitution method directly.