Explanation:
the original pool area is the area of a rectangle :
length × width = 25 × 10 = 250 ft²
when doubling the area that means the new area is
250 × 2 = 500 ft²
as you can see that is NOT doubling both length and width. only one of them ( e.g. 25 × 20 or 50 × 10).
or changing them to any (length, width) pair so that
length × width = 500
e.g. making the length 30 ft means for the width :
width = 500/length = 500/30 = 50/3 = 16.6666... ft