Per day work done by A – 1/3
Per day work done by B – 1/5
Per day work done by C – 1/6
Now, suppose A, B , C together would take x days to complete the work.
Then, per day work done by A,B and C together = (1/3 + 1/5 + 1/6) = 1/x
Solving for x – 1/x = (10+6+5)/(6*5)
x = 30/21 = 10/7 = 1 3/7 days.
Ans : Option A.