There are two pumps to fill a tank with water. First pump can fill the empty tank in 8 hours, while the second in 10 hours, If both the pumps are opened at the same time and kept open for 4 hours, the part of the tank that will be filled up is :

There are two pumps to fill a tank with water. First pump can fill the empty tank in 8 hours, while the second in 10 hours, If both the pumps are opened at the same time and kept open for 4 hours, the part of the tank that will be filled up is :
[A] $\frac{2}{5}$
[B] $\frac{1}{5}$
[C] $\frac{9}{10}$
[D] $\frac{1}{10}$

$\frac{9}{10}$
Part of the tank filled in an hour by both pumps
$= \frac{1}{8}+\frac{1}{10} = \frac{5+4}{40} = \frac{9}{40}$
∴ Part of the tank filled in 4 hours
$= \frac{4\times 9}{40} = \frac{9}{10}$
Hence option [C] is correct answer.