A, B and C can do a piece of work in 20 days, 30 days and 60 days. If A works continuously and on every third day B and C also work with A, then in how many days the work will be completed???

work done by a b and c together is 1/10
work done by a is 1/20
so work done in three days is
1/20+1/20+1/10 = 4/20 = 1/5
So work will be completed in 15 Days

ans .14 days
work done by b and c in one day is 1/30 +1/60 = 1/20.
work done by a in one day =1/20.
they for one day for every 3 days
work done by a is 20 ===> 20/3 is 6 days.
b and c work for 6 days .
therefor 20-6 =
14

Please explain how????

15 days