If one pipe A can fill a tank in 20 minutes, then 5 pipes, each of 20% efficiency of A, can fill the tank in:

Answer: Option C

Efficiency of pipe A,
$$\frac{100}{20}$$ = 5%
20% of efficiency of A = 1%
Then, efficiency of 5 such pipes = 5%.
Then,
Time taken to fill the tank = $$\frac{100}{5}$$ = 20 min.