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Q.

A man can do a piece of work in 5 days, but with the help of his son, he can do it in 3 days. In what time can the son do it alone?


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a

7.5 days

b

8.5 days

c

9.5 days

d

11.5 days 

answer is A.

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Detailed Solution

Given,
Number of days required to do piece of work by man = 5 days
Man and his son both done the work = 3 days
Total work (LCM of 5 and 3) = 5 × 3 = 15 days
We have,
Work = time × efficiency
Now, calculation
1st case for man
Efficiency = 15/5 = 3
2nd case for man + son
Efficiency = 15/3 = 5
Thus,
Efficiency of son=(efficiency of Man + son) - efficiency of man 
Efficiency of son =5-3=2
Therefore the son alone can do the work,
152=7.5 Days
Correct option is 1.
 
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