A person purchases 90 clocks and sells 40 clocks at a gain of 10% and 50 clocks at a gain of 20%. If he sold all of them at a uniform profit of 15%, then he would have got Rs. 40 less. The cost price of each clock is:
Rs. 80
Let C.P. of each clock be Rs. x.
Then, C.P. of 90 clocks = Rs. 90x.
∴ [(110 % of 40x) + (120% of 50x)] - (115% of 90x) = 40
⟹ 44x + 60x - 103.5x = 40 ⟹ 0.5x = 40 ⟹ x = 80.