| ( | 140 | − | a | − | 100 | ) | ( | 20 | + | 5 | a | ) | = | 1250 |
| 方程左边 = | 140 | ( | 20 | + | 5 | a | ) | − | a | ( | 20 | + | 5 | a | ) | − | 100 | ( | 20 | + | 5 | a | ) |
| = | 140 | × | 20 | + | 140 | × | 5 | a | − | a | ( | 20 | + | 5 | a | ) | − | 100 | ( | 20 | + | 5 | a | ) |
| = | 2800 | + | 700 | a | − | a | ( | 20 | + | 5 | a | ) | − | 100 | ( | 20 | + | 5 | a | ) |
| = | 2800 | + | 700 | a | − | a | × | 20 | − | a | × | 5 | a | − | 100 | ( | 20 | + | 5 | a | ) |
| = | 2800 | + | 680 | a | − | a | × | 5 | a | − | 100 | ( | 20 | + | 5 | a | ) |
| = | 2800 | + | 680 | a | − | a | × | 5 | a | − | 100 | × | 20 | − | 100 | × | 5 | a |
| = | 2800 | + | 680 | a | − | a | × | 5 | a | − | 2000 | − | 500 | a |
| = | 800 | + | 180 | a | − | a | × | 5 | a |
| 800 | + | 180 | a | − | a | × | 5 | a | = | 1250 |