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.