Let x be the number of hours taken to fill the pool.
Using proportion;
1/7 pool= 1/10 hour
1 pool = x
cross-multiply
1/7 x = 1/10
=
cross- multiply
10x = 7
Divide both-side of the equation by 10
=
(On the left-hand side of the equation 10 at the numerator will cancel-out 10 at the denominaor while at the right-hand side of the equation 7 will be divided by 10)