| r | × | 8 100 | + | ( | 1 | − | r | ) | × | 13 100 | = | 16 100 |
| Left side of the equation = | 8 100 | r | + | 1 | × | 13 100 | − | r | × | 13 100 |
| = | 8 100 | r | + | 13 100 | − | r | × | 13 100 |
| = | - | 1 20 | r | + | 13 100 |
| - | 1 20 | r | + | 13 100 | = | 16 100 |
| - | 1 20 | r | = | 16 100 | − | 13 100 |
| - | 1 20 | r | = | 3 100 |
| - | 3 100 | = | 1 20 | r |
1 20 | r | = | - | 3 100 |
| r | = | - | 3 100 | ÷ | 1 20 |
| = | - | 3 100 | × | 20 |
| = | - | 3 5 | × | 1 |
| r | = | - | 3 5 |
| r | = | - 0.6 |