A person purchases 90 watches and sells 40 watches at a gain of 10% and 50 watches at a gain of 20%. If he sold all of them at a uniform profit of 15%, then he would have got $ 40 less. The cost price of each clock is: Choose the most appropriate option:
A) 70
B) 90
C) 80
D) 60
E) 50
C   80   Let C.P. of clock be $ x.   Then, C.P. of 90 watches = $ 90x.   [(110% of 40x) + (120% of 50x)] - (115% of 90x) = 40   44x + 60x - 103.5x = 40   0.5x = 40 =&gt; x = 80
<result>C</result>
A person purchases 90 watches and sells 40 watches at a gain of 10% and 50 watches at a gain of 20%. If he sold all of them at a uniform profit of 15%, then he would have got $ 40 less. The cost price of each clock is: