A person purchases 90 clocks and sells 40 clocks at again of 10% and 50 clocks at 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:

Let each clock C.P. be x

So , we can say that cost of 90 clocks will be 90x

He sells 40 clocks at a gain of 10% i.e. 110% and 50 clocks at a gain of 20% i.e. 120%

S.P. of 90 clocks = [(40x of 110%) + (50x of 120%)]

                            =

                            =

                            =

                            =

Now , he sold all the clocks at a uniform profit 15% i.e. 115% he would have got Rs 40 less.

S.P of 90 clocks(after uniform profit of 15% and also got 40 less) so,

104x - (90x of 115%) - 40 = 0