I believe your answer is the first option, 1/2 ft.
*Volume is expressed by length x width x height.
If we multiply the length 5/7 and the height 1 & 1/2 we get 15/14:
*To multiply fractions you must multiply across, multiply the numerator (or top number) by the numerator and the denominator (bottom number) by the denominator.
- Let's turn 1 & 1/2 into a mixed fraction to make things a little easier. 1 & 1/2 also equivalent to 3/2
-Multiply the numerators together, 5 from the 5/7 and now 3 from the 3/2
(5 x 3 = 15)
-Multiply the denominators together, 7 from the 5/7 and 2 from the 3/2
(7 x 2 = 14)
Therefore, 5/7 x 1 & 1/2 is 15/14.
Now what multiplied by 15/14 equals 15/28?
Well... 1 x 15 = 15. That gives us our numerator, 1.
2 x 14 = 28... and we have our denominator 2.
Putting those two together we get a fraction of 1/2. Your width is then 1/2 ft.
I'm so sorry for all the work here! I know it's a lot. I hope it's not too confusing to look at. Hopefully this helps!