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

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