Shruti takes 10 days to finish a piece of work while Shankar takes 15 days to finish the same piece of work. Harish works twice as fast as Shruti. How many days will all three of them together take to finish the same piece of work ?

Explanation:

Time taken by Shruti in doing the work = 10 days

The efficiency of Harish is twice to that of Shruti.

∴ Time taken by Harish

= 5 days

Time taken by Shankar

= 15 days

∴ Work done by all three in 1 day

= 1\10+1\5+1\15

3+6+2\30 = 11\30

Hence, the work will be finished in 30\11days.