| 400 | × | 10 | ( | 1 | + | a | ) | × | 12 5 | + | 500 | × | 10 | ( | 1 | + | a | ) | × | 12 5 | = | 21600 | ( | 1 | + | 20 | ÷ | 9 | × | a | ) |
| Left side of the equation = | 9600 | ( | 1 | + | a | ) | + | 12000 | ( | 1 | + | a | ) |
| 9600 | ( | 1 | + | a | ) | + | 12000 | ( | 1 | + | a | ) | = | 21600 | ( | 1 | + | 20 | ÷ | 9 | × | a | ) |
| Left side of the equation = | 9600 | × | 1 | + | 9600 | a | + | 12000 | ( | 1 | + | a | ) |
| = | 9600 | + | 9600 | a | + | 12000 | ( | 1 | + | a | ) |
| = | 9600 | + | 9600 | a | + | 12000 | × | 1 | + | 12000 | a |
| = | 9600 | + | 9600 | a | + | 12000 | + | 12000 | a |
| = | 21600 | + | 21600 | a |
| 21600 | + | 21600 | a | = | 21600 | ( | 1 | + | 20 | ÷ | 9 | × | a | ) |
| Right side of the equation = | 21600 | × | 1 | + | 21600 | × | 20 | ÷ | 9 | × | a |
| = | 21600 | + | 48000 | a |
| 21600 | + | 21600 | a | = | 21600 | + | 48000 | a |
| 21600 | a | − | 48000 | a | = | 21600 | − | 21600 |
| - | 26400 | a | = | 21600 | − | 21600 |
| - | 26400 | a | = | 0 |
| - | 0 | = | 26400 | a |
| 26400 | a | = | - | 0 |
| a | = | - | 0 | ÷ | 26400 |
| = | - | 0 | × | 1 26400 |
| a | = | 0 |