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 _______ ?

A. Rs.80
B. Rs.90
C. Rs.50
D. Rs.60

Let C.P. of 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