Explanation:

Solution: 
On 1st day A completes 1/20 of work.
On 2nd day, A and B together completes 1/20 + 1/30 = 1/12 of work.
Using LCM method:
Total work = LCM of 20 and 12 = 60
So efficiency of A = 60/20 = 3.
And efficiency of A and B on 2nd day = 60/12 = 5
So in 2 days they complete 3+5 = 8 works
Multiply by 7 both sides.
In 14 days they complete 56 works.
Work left = 60 – 56 = 4
Now on 15th days A, turn. He completes 3 of the work out if 4 left.
Now remaining work = 4 – 3 = 1.
On 16th day, A and B’s turn. So they complete remaining 1 work in 1/5 day.
So total 15 1/5 days.