Working alone, two pipes A and B require 9 hours and 6.25 hours more respectively to fill a pool than if they were working together. Find the total time taken to fill the pool if both were working together.

**(A)** 6

**(B)** 6.5

**(C)** 7

**(D)** 7.5

**Answer:** **(D)** **Explanation:** Let the time taken if both were working together be ‘n’ hours.

=> Time taken by A = n + 9

=> Time taken by B = n + 6.25

In such kind of problems, we apply the formula :

n^{2} = a x b, where ‘a’ and ‘b’ are the extra time taken if both work individually than if both work together.

Therefore, n^{2} = 9 x 6.25

=> n = 3 x 2.5 = 7.5

Thus, working together, pipes A and B require 7.5 hours.

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