| 4200 | × | 2 | ( | 80 | − | t | ) | = | 4200 | × | 1 2 | ( | t | − | 10 | ) |
| Left side of the equation = | 8400 | ( | 80 | − | t | ) |
| 8400 | ( | 80 | − | t | ) | = | 4200 | × | 1 2 | ( | t | − | 10 | ) |
| Left side of the equation = | 8400 | × | 80 | − | 8400 | t |
| = | 672000 | − | 8400 | t |
| 672000 | − | 8400 | t | = | 4200 | × | 1 2 | ( | t | − | 10 | ) |
| Right side of the equation = | 2100 | ( | t | − | 10 | ) |
| 672000 | − | 8400 | t | = | 2100 | ( | t | − | 10 | ) |
| Right side of the equation = | 2100 | t | − | 2100 | × | 10 |
| = | 2100 | t | − | 21000 |
| 672000 | − | 8400 | t | = | 2100 | t | − | 21000 |
| - | 8400 | t | − | 2100 | t | = | - | 21000 | − | 672000 |
| - | 10500 | t | = | - | 21000 | − | 672000 |
| - | 10500 | t | = | - | 693000 |
| 693000 | = | 10500 | t |
| 10500 | t | = | 693000 |
| t | = | 693000 | ÷ | 10500 |
| = | 693000 | × | 1 10500 |
| = | 66 | × | 1 |
| t | = | 66 |