Will had 13 more files than Carl.

If Carl has 1 unit, then

2 units = 215-13 = 202

1 unit = 101

Carl had 101 files and Will had 101 + 13 = 114 files.

Given Carl sold 10% less files than Will, if Will sold 10u, then Carl sold 9u.

Will left with 114- 10u and Carl left with 101 – 9u

Also given Carl had half as many files left as Will,

101 – 9u = ½ * ( 114 – 10u)

Multiple 2 on both sides,

202 – 18u = 114 – 10u

8u = 88

U = 11

114 – 10(11) = 4

Will left with 4 files.