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 option.
A) Rs. 360
B) Rs. 120
C) Rs. 240
D) Rs. 300
E) None
Solution:   In one day X can finish 1/15th of the work.   In one day Y can finish 1/10th 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 = 1/5th of the work.   Therefore, 1/Z = 1/30.   Ratio of their efficiencies = 1/15: 1/10: 1/30 = 2: 3: 1.Therefore Z receives 1/6th of the total money.   According to their efficiencies money is divided as 240: 360: 120.   Hence, the share of Z = Rs. 120.   Answer Rs. 120
<result>B</result>
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?