| ( | 15 | − | y | ) | ( | 5 | y | + | 400 | ) | = | 4250 |
| Left side of the equation = | 15 | ( | 5 | y | + | 400 | ) | − | y | ( | 5 | y | + | 400 | ) |
| = | 15 | × | 5 | y | + | 15 | × | 400 | − | y | ( | 5 | y | + | 400 | ) |
| = | 75 | y | + | 6000 | − | y | ( | 5 | y | + | 400 | ) |
| = | 75 | y | + | 6000 | − | y | × | 5 | y | − | y | × | 400 |
| = | - | 325 | y | + | 6000 | − | y | × | 5 | y |
| - | 325 | y | + | 6000 | − | y | × | 5 | y | = | 4250 |