Correct Answer - Option 1 : 20 hours

__Given:__

Time taken by (L + K) to fill the tank = 4 hours

Time taken by K to fill the tank = 5 hours

**Formula Used:**

Efficiency = Total work/Time taken

__Calculation:__

LCM of 4 and 5 = 20 = Total work

Efficiency of L and K = 20/4 units/day

⇒ 5 units/day

Efficiency of K = 20/5 units/day

⇒ 4 units/day

Efficiency of L = (5 – 4) = 1 unit/day

Time taken by pipe L = 20/1 hours

⇒ 20 hours

**∴ Time taken by pipe L alone is 20 hours**