Question

One pump working continuously can fill a reservoir in thirty days. A second pump can fill the reservoir in twenty days. How long would it take both pumps working together to fill the reservoir?

Answer 1

• One pump will fill a reservoir of volume $v$ in $30$ days , So In $1$ day it can fill $\frac{v}{30}$
• Second pump can fill a reservoir of volume $v$ in $20$ days , So in $1$ day it can fill $\frac{v}{20}$
• Therefore they both combined can fill $\frac{v}{30}+\frac{v}{20}=\frac{v}{12}$ in $12$ day
• So They both combined will take $12$ days to fill a reservoir of volume $v$