| 4200 | = | ( | 15 | − | x | ) | ( | 400 | + | 4 | x | ) |
| Right side of the equation = | 15 | ( | 400 | + | 4 | x | ) | − | x | ( | 400 | + | 4 | x | ) |
| = | 15 | × | 400 | + | 15 | × | 4 | x | − | x | ( | 400 | + | 4 | x | ) |
| = | 6000 | + | 60 | x | − | x | ( | 400 | + | 4 | x | ) |
| = | 6000 | + | 60 | x | − | x | × | 400 | − | x | × | 4 | x |
| = | 6000 | − | 340 | x | − | x | × | 4 | x |
| 4200 | = | 6000 | − | 340 | x | − | x | × | 4 | x |