1)

Two pipes C and D can fill a tank in 36 hours and 45 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?

A) 40 hours

B) 50 hours

C) 60 hours

D) 20 hours

E) none of the above

Option D

### Explanation:

Part filled by $C$ in $1$ hour $=\frac{1}{36}$

Part filled by $D$ in $1$ hour $=\frac{1}{45}$

Part filled by $(C+D)$ in $1$ hour $=\left(\frac{1}{36}+\frac{1}{45}\right)$ $=\frac{9}{80}$ $=\frac{1}{20}$

Hence, both the pipes together will fill the tank in 20 hours.