| 420 | ( | 816 | − | t | ) | = | 4200 | × | 5 | ( | t | − | 20 | ) |
| Left side of the equation = | 420 | × | 816 | − | 420 | t |
| = | 342720 | − | 420 | t |
| 342720 | − | 420 | t | = | 4200 | × | 5 | ( | t | − | 20 | ) |
| Right side of the equation = | 21000 | ( | t | − | 20 | ) |
| 342720 | − | 420 | t | = | 21000 | ( | t | − | 20 | ) |
| Right side of the equation = | 21000 | t | − | 21000 | × | 20 |
| = | 21000 | t | − | 420000 |
| 342720 | − | 420 | t | = | 21000 | t | − | 420000 |
| - | 420 | t | − | 21000 | t | = | - | 420000 | − | 342720 |
| - | 21420 | t | = | - | 420000 | − | 342720 |
| - | 21420 | t | = | - | 762720 |
| 762720 | = | 21420 | t |
| 21420 | t | = | 762720 |
| t | = | 762720 | ÷ | 21420 |
| = | 762720 | × | 1 21420 |
| = | 1816 | × | 1 51 |
| t | = | 1816 51 |
| t | = | 35.607843 |