A shopkeeper buys two books for Rs.300. He sells the first book at a profit of 20% and the second book at a loss of 10%. What is the selling price of the first book, if, in the whole transaction there is no profit no loss?

Rs.100

Using allegation method,

20%                  10%

              0%

10          :           20

= 1         :           2

1 + 2 = 3 unit

= 3 unit = 300

= 1 unit = 100

CP of the first book = 100