| 3 | × | 4200 | ( | a | − | 20 | ) | = | 2 | × | 4200 | ( | 80 | − | a | ) |
| Left side of the equation = | 12600 | ( | a | − | 20 | ) |
| 12600 | ( | a | − | 20 | ) | = | 2 | × | 4200 | ( | 80 | − | a | ) |
| Left side of the equation = | 12600 | a | − | 12600 | × | 20 |
| = | 12600 | a | − | 252000 |
| 12600 | a | − | 252000 | = | 2 | × | 4200 | ( | 80 | − | a | ) |
| Right side of the equation = | 8400 | ( | 80 | − | a | ) |
| 12600 | a | − | 252000 | = | 8400 | ( | 80 | − | a | ) |
| Right side of the equation = | 8400 | × | 80 | − | 8400 | a |
| = | 672000 | − | 8400 | a |
| 12600 | a | − | 252000 | = | 672000 | − | 8400 | a |
| 12600 | a | + | 8400 | a | = | 672000 | + | 252000 |
| 21000 | a | = | 672000 | + | 252000 |
| 21000 | a | = | 924000 |
| a | = | 924000 | ÷ | 21000 |
| = | 924000 | × | 1 21000 |
| = | 44 | × | 1 |
| a | = | 44 |