A and B can finish a work 30 days if they work together. They worked together for 20 days and then B left. A finished the remaining work in another 20 days. In how many days A alone can finish the work? Choose the correct answer
A) 30
B) 60
C) 40
D) 20
E) 50

Rationale: amount of work done by a and b in 1 day = 1 / 30 amount of work done by a and b in 20 days = 20 × ( 1 / 30 ) = 20 / 30 = 2 / 3 remaining work – 1 – 2 / 3 = 1 / 3 a completes 1 / 3 work in 20 days amount of work a can do in 1 day = ( 1 / 3 ) / 20 = 1 / 60 = > a can complete the work in 60 days answer is b .
Answer: B