In the proportional theory,
28 units is the total measurement that includes 8 units.
So, 10 + 3x - 5 is the total measurement that includes 10 units proportionally.
This means,
28 = 10 + 3x - 5
Now solve for x,
28 = 10 + 3x - 5
28 - 10 = 3x - 5
18 + 5 = 3x
23 = 3x
23 / 3 = x
So x = 23/3 or 7.6666....
