If you would like to write the expression in math, you can do this using the following steps:
one fourth the sum of r and 10 ... 1/4 * (r + 10)
is identical to ... =
r minus 4 ... r - 4
1/4 * (r + 10) = r - 4
1/4 * r + 1/4 * 10 = r - 4
10/4 + 4 = r - 1/4 * r
10/4 + 16/4 = 4/4 * r - 1/4 * r
26/4 = 3/4 * r /*4/3
r = 26/4 * 4/3
r = 26/3
r = 8 2/3
The correct result would be 8 2/3.