| ( | 15 | + | 21 | ) | × | 4 | + | ( | 15 | + | 24 | ) | ( | X | − | 4 | ) | = | 720 |
| 方程左边 = | 15 | × | 4 | + | 21 | × | 4 | + | ( | 15 | + | 24 | ) | ( | X | − | 4 | ) |
| = | 60 | + | 84 | + | ( | 15 | + | 24 | ) | ( | X | − | 4 | ) |
| = | 144 | + | ( | 15 | + | 24 | ) | ( | X | − | 4 | ) |
| = | 144 | + | 15 | ( | X | − | 4 | ) | + | 24 | ( | X | − | 4 | ) |
| = | 144 | + | 15 | X | − | 15 | × | 4 | + | 24 | ( | X | − | 4 | ) |
| = | 144 | + | 15 | X | − | 60 | + | 24 | ( | X | − | 4 | ) |
| = | 84 | + | 15 | X | + | 24 | ( | X | − | 4 | ) |
| = | 84 | + | 15 | X | + | 24 | X | − | 24 | × | 4 |
| = | 84 | + | 15 | X | + | 24 | X | − | 96 |
| = | - | 12 | + | 39 | X |
| - | 12 | + | 39 | X | = | 720 |
| 39 | X | = | 720 | + | 12 |
| 39 | X | = | 732 |
| X | = | 732 | ÷ | 39 |
| = | 732 | × | 1 39 |
| = | 244 | × | 1 13 |
| X | = | 244 13 |
| X | = | 18.769231 |