| 1800 | = | ( | 80 | − | 4 | a | ) | ( | 60 | − | 2 | a | ) |
| 方程右边 = | 80 | ( | 60 | − | 2 | a | ) | − | 4 | a | ( | 60 | − | 2 | a | ) |
| = | 80 | × | 60 | − | 80 | × | 2 | a | − | 4 | a | ( | 60 | − | 2 | a | ) |
| = | 4800 | − | 160 | a | − | 4 | a | ( | 60 | − | 2 | a | ) |
| = | 4800 | − | 160 | a | − | 4 | a | × | 60 | + | 4 | a | × | 2 | a |
| = | 4800 | − | 160 | a | − | 240 | a | + | 8 | a | a |
| = | 4800 | − | 400 | a | + | 8 | a | a |
| 1800 | = | 4800 | − | 400 | a | + | 8 | a | a |