Two pipes A and B can fill a tank in 20 and 30 minutes respectively. If both the pipes are used together, then how long will it take to fill the tank?

Answer: Option A

Part filled by A in 1 minute = $$\frac{1}{20}$$
Part filled by B in 1 minute = $$\frac{1}{30}$$
Part filled by (A + B) in 1 minute
$$\begin{array}{rl}& =\frac{1}{20}+\frac{1}{30}=\frac{1}{12}\end{array}$$
∴ Both pipes can fill the tank in 12 minutes