This question was previously asked in

UP Police SI (दरोगा) Previous Paper 10 (Held On: 13 Dec 2017 Shift 1)

Option 3 : 990 cubic feet

**Given:**

Lamplit has to light 100 gas lamps.

It takes 60 seconds to travel from one lamppost to the next.

Each lamp burns 12 cubic feet of gas per minute.

**Calculation:**

Each lamp burns 12 cubic feet of gas per hour

In one minute = 12/60

⇒ 1/5 cubic feet gas per minute

According to the question,

He takes 1 min. to go from one lamp post to the other.

When he lights the last lamp post, the first lamppost will burn for 99 minutes, the second for 98 minutes, so on

Gas used = (1/5)(99 + 98 + 97 + ....... + 1)

⇒ (100 × 99)/(2 × 5)

⇒ 990 cubic feet.

**∴ Gas will be burnt by the time he lights the last 990 cubic feet**.