A sells an article to B at 20% profit. B sells it to C at 15% loss. If C pays Rs 510 for it, then A purchased it at:

A bought it for Rs 500.

Let A's cost = x; B pays 1.20x at 20% profit.

C pays 1.20x x 0.85 at 15% loss = 1.02x.

1.02x = 510.

x = 500, so A's cost = Rs 500.