| 300 | ( | n | + | 1 | ) | n | ÷ | 2 | + | 1300 | n | = | 72000 |
| Left side of the equation = | 150 | ( | n | + | 1 | ) | n | + | 1300 | n |
| 150 | ( | n | + | 1 | ) | n | + | 1300 | n | = | 72000 |
| Left side of the equation = | 150 | n | n | + | 150 | × | 1 | n | + | 1300 | n |
| = | 150 | n | n | + | 150 | n | + | 1300 | n |
| = | 150 | n | n | + | 1450 | n |
| 150 | n | n | + | 1450 | n | = | 72000 |