Dan had a roll of wrapping paper. he cut the paper from the roll into 15 equal pieces. each piece was yard long.

what was the original length of the wrapping paper on the roll?

a.2 yards

b.5 yards

c.10 yards

d.15 yards

D. 15 yards
This can be explained simply, or with algebra for a more indepth view:
Simply:
If something is cut into 15 equal pieces and each piece is 1 yard long, if we wanted to find out the original length: add them all back together.
1+1+1+1+1+1+1+1+1+1+1+1+1+1 = 15 yards
or more simply done by 1*15 = 15 yards

With algebra:
x/15 yards = 1 yards
x = 15 yards to start with