Two pipes A and B can fill a cistern in 30 and 45 hours respectively. If both the pipes are opened together when the tank is empty then the time (in hours ( it will take respectively to fill the tank is :

18

The correct answer is Option (4) → 18

Combined Rate = Rate of A + Rate of B

$=\frac{1}{30}+\frac{1}{45}$

$=\frac{3+2}{40}=\frac{5}{90}$

$=\frac{1}{18}$

i.e. in one hour both pipes filled up $\frac{1}{18}th$of tank per hour.

Therefore,

Pipes A and B together will take 18 hours to fill the cisterns.