A merchant buys p apples for ₹ q and sells q apples for ₹ p. If p < q, then in the whole outlay, he makes.

Given,
CP of p apples = q
∴ CP of 1 apple = q/p
and SP of q apples = p
∴ SP of 1 apple = p/q
Given, p < q
∴ Loss = q/p - p/q = (q² - p²) / pq
And Loss% = (Loss/CP) x 100
= {(q² - p²)/pq} /(q/p)] x 100
= (q² - p²)/q² x 100
Therefore, when p < q , then the person had a loos which is given by
100 x (q² - p²/q²)% loss.