| 120 | + | 2 | r | + | 120 | + | 120 | + | 1 2 | ( | r | − | 30 | ) | = | 360 |
| Left side of the equation = | 360 | + | 2 | r | + | 1 2 | ( | r | − | 30 | ) |
| 360 | + | 2 | r | + | 1 2 | ( | r | − | 30 | ) | = | 360 |
| Left side of the equation = | 360 | + | 2 | r | + | 1 2 | r | − | 1 2 | × | 30 |
| = | 360 | + | 2 | r | + | 1 2 | r | − | 15 |
| = | 345 | + | 5 2 | r |
| 345 | + | 5 2 | r | = | 360 |
5 2 | r | = | 360 | − | 345 |
5 2 | r | = | 15 |
| r | = | 15 | ÷ | 5 2 |
| = | 15 | × | 2 5 |
| = | 3 | × | 2 |
| r | = | 6 |