Correct Answer - Option 4 : 30 hours

**Given:**

A fills a tank in 12 hours

B empties the same tank in 20 hours

Both taps are open together.

**Concept used:**

Total days = (total work)/efficiency

**Explanation:**

LCM of 12 and 20 is 60

Let the total capacity of the tank be = 60 litre

Efficiency of A = + 5 liter/hour

Efficiency of B = - 3 liter/hour

Both A + B can fill the tank in 60/(5 - 3) hours.

⇒ 60/2 = 30 hours

**∴ The tank will fill in 30 hours.**