Question

the selling price of 18 books is equal to the cost price of 21 books find the gain or loss percent​

Answer 1

Let the cost price of 21 books be a

Cost price of one book = a/21

Selling price of 18 book = a

=> Selling price of 1 book = a / 18

Gain on one book = SP - CP

= a/18 - a/21

= ( 7a - 6a) / 126

= a / 126.

Now,

Gain % = Gain on 1 book × 100 / CP of 1 book

= ( a / 126) × 100 / ( a / 21)

= 100 × 21 / 126

= 100 / 6

= 16.67 %