No she would not have a fuller tank if she had 4/5 of one instead of 7/8.
In this situation you need to convert the fractions so that they have common denominators.
In order to achieve this I multiplied both the numerator and the denominator of 7/8 by 5.
I also multiplied both the numerator and denominator of 4/5 by 8.
7/8*5=35/40.
4/5*8=32/40.
Now you can easily tell that 7/8 is larger than 4/5.
Hope this helps! (: