| ( | 58 | − | y | − | 35 | ) | ( | 400 | + | 20 | y | ) | = | 8400 |
| 方程左边 = | 58 | ( | 400 | + | 20 | y | ) | − | y | ( | 400 | + | 20 | y | ) | − | 35 | ( | 400 | + | 20 | y | ) |
| = | 58 | × | 400 | + | 58 | × | 20 | y | − | y | ( | 400 | + | 20 | y | ) | − | 35 | ( | 400 | + | 20 | y | ) |
| = | 23200 | + | 1160 | y | − | y | ( | 400 | + | 20 | y | ) | − | 35 | ( | 400 | + | 20 | y | ) |
| = | 23200 | + | 1160 | y | − | y | × | 400 | − | y | × | 20 | y | − | 35 | ( | 400 | + | 20 | y | ) |
| = | 23200 | + | 760 | y | − | y | × | 20 | y | − | 35 | ( | 400 | + | 20 | y | ) |
| = | 23200 | + | 760 | y | − | y | × | 20 | y | − | 35 | × | 400 | − | 35 | × | 20 | y |
| = | 23200 | + | 760 | y | − | y | × | 20 | y | − | 14000 | − | 700 | y |
| = | 9200 | + | 60 | y | − | y | × | 20 | y |
| 9200 | + | 60 | y | − | y | × | 20 | y | = | 8400 |