Option 1 : 45 days

**Given:**

A and B together finish work = 30 days

B left the work = 20 days after working together

Remaining work finished by A = 15 days

**Formula used:**

Efficiency = Total work/Time

**Calculation:**

Let the total work done by A and B be 30 units

According to the question

Total work done by A and B in 20 days = 20 units

Remaining work = (30 - 20) units = 10 units

Since, B left after 20 days and remaining work completed by A alone in 15 days

Efficiency of A = 10/15 = (2/3) units per day

Time taken by A to finish job alone = [30/(2/3)]

⇒ (30 × 3)/2

⇒ 45 days

**∴ A can alone finish the job in 45 days.**