Answer:
7a and 7b are true, 7c is false.
Explanation:
The image shows that she has 9/10 meters of the string available.
She wants to use 3/5 meters, so the part remaining is

Basically, the statement 7a is true, because with that difference we can find the string remaining. Also, with this results, we can say that 7c is false, becasue 1/5 meters is not the string remaining, it's 3/10.
Additionally, 7b is true, because 3/5 and 6/10 are actually equivalent factors, observe that if you multiply the first fraction by two, you'll get the second fraction.