The C.P of 15 books is equal to the S.P of 18 books. Find the gain% or loss% ?

Let Cost Price per book = 1
Cost of 15 books = 15
15 is obtained by selling 18 books
So Selling Price per book = 15 / 18
CP – SP = Loss
= 1 − 15 / 18
= 1 / 6
Percentage = 1 / 6 ∗ 100
= 16 2/3 % Loss