| ( | 40 | − | x | ) | ( | 20 | + | 3 | x | ) | = | 1600 |
| Left side of the equation = | 40 | ( | 20 | + | 3 | x | ) | − | x | ( | 20 | + | 3 | x | ) |
| = | 40 | × | 20 | + | 40 | × | 3 | x | − | x | ( | 20 | + | 3 | x | ) |
| = | 800 | + | 120 | x | − | x | ( | 20 | + | 3 | x | ) |
| = | 800 | + | 120 | x | − | x | × | 20 | − | x | × | 3 | x |
| = | 800 | + | 100 | x | − | x | × | 3 | x |
| 800 | + | 100 | x | − | x | × | 3 | x | = | 1600 |
| x1= | 40 3 |
| x2=20 |