A can do ¼ of work in = 5 days
A can do 1 work in = 5 × 4 days
= 20 days
B can do 1/3 of work in = 10 days
B can do 1 work in = 10 × 3 days
= 30 days
A’s 1 day work = 1/20
B’s 1 day work = 1/30
(A + B)’s 1 day work = (1/20) + (1/30)
= (3 + 2)/60
= 5/60
= 1/12
∴ A and B working together will complete the work in = 12 days