To solve this problem you must apply the proccedure shown below:
1. In a hour you have [tex] \frac{1}{38} [/tex] of the pool filled and [tex] \frac{1}{40} [/tex].Taking [tex] \frac{1}{4} [/tex] of the pool filled, you have the following expression:
[tex] \frac{1}{38}-\frac{1}{40}=\frac{1}{4}/t [/tex]
2. Solve for [tex] t [/tex], as following:
[tex] t=\frac{760}{4}\\ t=190 hours [/tex]
The answer is: [tex] 190 hours [/tex]
