Explanation:
I guess, she is checking how FAR she runs each day (and not how fast ...).
how many more miles did she run on Friday ?
we get this by calculating the difference between the 2 values. that is what subtraction is for.
7 2/8 - 3 3/4
as long as we have the numbers as combination of full and fraction numbers, we can subtract the full numbers right away.
7 2/8 - 3 3/4 = 4 2/8 - 3/4
now we convert both numbers to full fractions :
4 2/8 - 3/4 = (32 + 2)/8 - 3/4 = 34/8 - 3/4
and then we bring both fractions to the same denominator, in this case 1/8 :
34/8 - 3/4 = 34/8 - 3/4×2/2 = 34/8 - 6/8 = 28/8
now we can simplify
28/8 = 7/2 = 3 1/2
so, she ran 3 1/2 miles more in Friday than on Tuesday.
FYI
I just wanted to show you the full formal flow of such a calculation.
but we could have gotten the result quicker by recognizing that we can simplify the numbers already at the beginning :
7 2/8 - 3 3/4 = 7 1/4 - 3 3/4 = 4 1/4 - 3/4 = 3 2/4 = 3 1/2