| 720 | ( | 1 2 | a | + | 400 | ) | ÷ | 560 | + | 50 | a | ÷ | 60 | = | 3 2 | a | + | 400 |
| 方程左边 = | 9 7 | ( | 1 2 | a | + | 400 | ) | + | 5 6 | a |
9 7 | ( | 1 2 | a | + | 400 | ) | + | 5 6 | a | = | 3 2 | a | + | 400 |
| 方程左边 = | 9 7 | × | 1 2 | a | + | 9 7 | × | 400 | + | 5 6 | a |
| = | 9 14 | a | + | 3600 7 | + | 5 6 | a |
| = | 31 21 | a | + | 3600 7 |
31 21 | a | + | 3600 7 | = | 3 2 | a | + | 400 |
31 21 | a | − | 3 2 | a | = | 400 | − | 3600 7 |
| - | 1 42 | a | = | 400 | − | 3600 7 |
| - | 1 42 | a | = | - | 800 7 |
800 7 | = | 1 42 | a |
1 42 | a | = | 800 7 |
| a | = | 800 7 | ÷ | 1 42 |
| = | 800 7 | × | 42 |
| = | 800 | × | 6 |
| a | = | 4800 |