X alone can do a piece of work in 15 days and Y alone can do it in 10 days. X and Y undertook to do it for Rs. 720. With the help of Z they finished it in 5 days. How much is paid to Z?
Choose the correct choice from the following choices:
A) rs . 360
B) rs . 120
C) rs . 240
D) rs . 300
E) none

Rationale: solution : in one day x can finish 1 / 15 th of the work . in one day y can finish 1 / 10 th of the work . let us say that in one day z can finish 1 / zth of the work . when all the three work together in one day they can finish 1 / 15 + 1 / 10 + 1 / z = 1 / 5 th of the work . therefore , 1 / z = 1 / 30 . ratio of their efficiencies = 1 / 15 : 1 / 10 : 1 / 30 = 2 : 3 : 1 . therefore z receives 1 / 6 th of the total money . according to their efficiencies money is divided as 240 : 360 : 120 . hence , the share of z = rs . 120 . answer b
Answer: B