A can do a piece of work in = 20 days
B can do a piece of work in = 15 days
∴ A’s 1 day work = 1/20
B’s 1 day work = 1/15
(A + B)’s 1 day work = 1/20 + 1/1 5
= (3 + 4)/60 = 7/60
(A + B)’s 6 day work = (7/60) × 6 = 7/10
Remaining work = 1 – (7/10)
= (10 – 7)/10 = 3/10
B can do 1 work in = 15 days
B can do 3/10 work in = 15 × (3/10) days
= 45/10 days = 9/2 days = 4. ½ days