| ( | n | − | 2 | ) | × | 180 | ÷ | n | = | 3 | ( | 180 | − | ( | n | − | 2 | ) | × | 180 | ÷ | n | ) | + | 20 |
| Multiply both sides of the equation by: | n |
| ( | n | − | 2 | ) | × | 180 | = | 3 | ( | 180 | − | ( | n | − | 2 | ) | × | 180 | ÷ | n | ) | n | + | 20 | n |
| n | × | 180 | − | 2 | × | 180 | = | 3 | ( | 180 | − | ( | n | − | 2 | ) | × | 180 | ÷ | n | ) | n | + | 20 | n |
| n | × | 180 | − | 2 | × | 180 | = | 3 | × | 180 | n | − | 3 | ( | n | − | 2 | ) | × | 180 | ÷ | n | × | n | + | 20 | n |
| n | × | 180 | − | 360 | = | 540 | n | − | 540 | ( | n | − | 2 | ) | ÷ | n | × | n | + | 20 | n |
| 180 | n | − | 360 | = | 560 | n | − | 540 | ( | n | − | 2 | ) | ÷ | n | × | n |
| 180 | n | − | 360 | = | 560 | n | − | 540 | n | ÷ | 1 | × | 1 | + | 540 | × | 2 | ÷ | 1 | × | 1 |
| 180 | n | − | 360 | = | 560 | n | − | 540 | n | + | 1080 |
| 180 | n | − | 360 | = | 20 | n | + | 1080 |
| 180 | n | − | 20 | n | = | 1080 | + | 360 |
| 160 | n | = | 1080 | + | 360 |
| 160 | n | = | 1440 |
| n | = | 1440 | ÷ | 160 |
| = | 1440 | × | 1 160 |
| = | 9 | × | 1 |
| n | = | 9 |