| ( | 200 | × | 120 | + | 3000 | ) | × | 30 100 | = | a | × | 120 | ( | 1 | − | 5 100 | ) | − | ( | 200 | × | 120 | + | 3000 | ) |
| 方程左边 = | 200 | × | 120 | × | 30 100 | + | 3000 | × | 30 100 |
| = | 7200 | + | 900 |
| = | 8100 |
| 8100 | = | a | × | 120 | ( | 1 | − | 5 100 | ) | − | ( | 200 | × | 120 | + | 3000 | ) |
| 方程右边 = | a | × | 120 | × | 1 | − | a | × | 120 | × | 5 100 | − | ( | 200 | × | 120 | + | 3000 | ) |
| = | a | × | 120 | − | a | × | 6 | − | ( | 200 | × | 120 | + | 3000 | ) |
| = | 114 | a | − | ( | 200 | × | 120 | + | 3000 | ) |
| = | 114 | a | − | 200 | × | 120 | − | 3000 |
| = | 114 | a | − | 24000 | − | 3000 |
| = | 114 | a | − | 27000 |
| 8100 | = | 114 | a | − | 27000 |
| - | 114 | a | = | - | 27000 | − | 8100 |
| - | 114 | a | = | - | 35100 |
| 35100 | = | 114 | a |
| 114 | a | = | 35100 |
| a | = | 35100 | ÷ | 114 |
| = | 35100 | × | 1 114 |
| = | 5850 | × | 1 19 |
| a | = | 5850 19 |
| a | = | 307.894737 |