Correct Answer - Option 4 : 45 days

Given:

No. of days required by A and B together = 30 days

Concept used:

Total work = No. of days × Efficiency of work

Calculation:

Let the whole work be 1.

⇒ (A + B) × 30 = 1

1 day efficiency of A and B together = 1/30

20 days work of A and B together = 20/30 = 2/3

Remaining work = 1 – 2/3 = 1/3

15 days work of A = 1/3

1 day work of A = 1/45

45 days work of A = 1

∴ A alone can do the entire work in 45 days.