| 21 | ( | 10 | − | a | ) | + | 20 | ( | 60 | − | 5 | a | ) | = | 1400 |
| Left side of the equation = | 21 | × | 10 | − | 21 | a | + | 20 | ( | 60 | − | 5 | a | ) |
| = | 210 | − | 21 | a | + | 20 | ( | 60 | − | 5 | a | ) |
| = | 210 | − | 21 | a | + | 20 | × | 60 | − | 20 | × | 5 | a |
| = | 210 | − | 21 | a | + | 1200 | − | 100 | a |
| = | 1410 | − | 121 | a |
| 1410 | − | 121 | a | = | 1400 |
| - | 121 | a | = | 1400 | − | 1410 |
| - | 121 | a | = | - | 10 |
| 10 | = | 121 | a |
| 121 | a | = | 10 |
| a | = | 10 | ÷ | 121 |
| = | 10 | × | 1 121 |
| a | = | 10 121 |
| a | = | 0.082645 |