Two pipes A and B can fill a tank in 5 hours and 8 hours respectively. Both the pipes are opened together. But, after 2 hours, pipe A is turned off. What is the total time required to fill the tank ?

A can be filled in a tank = 5 hours  $\Rightarrow$for 1 hour$\Rightarrow \frac{1}{5}$ part

B can be filled in a tank = 8 hours  for 1 hour  $\Rightarrow \frac{1}{8}$ part

A & B can fill the tank in 2 hours is

$\Rightarrow 2[\frac{1}{5}+\frac{1}{8}]=2\left[\frac{8+5}{40}\right]=\frac{13}{20}$ part

  $\begin{array}{l}\text{Remaining\hspace{0.17em}\hspace{0.17em}part}=1-\frac{13}{20}=\frac{7}{20}part\\ \frac{1}{8}partfilledbyBfor\Rightarrow 1hour\\ \frac{7}{20}partcanbefilledby\Rightarrow xhours\\ \frac{7}{20}\times 8=1\times x\\ \frac{14}{5}=x\end{array}$                               

x = 2.8 hrs

$\begin{array}{l}\therefore Totaltimerequiredtofill\text{\hspace{0.17em}\hspace{0.17em}tank}=2hrs+2.8hrs\\ \Rightarrow 4.8hrs=4hrs48\text{\hspace{0.17em}\hspace{0.17em}minutes}\end{array}$