| ( | 20 | − | x | ) | ( | 99 | + | 10 | x | ) | = | 1980 |
| Left side of the equation = | 20 | ( | 99 | + | 10 | x | ) | − | x | ( | 99 | + | 10 | x | ) |
| = | 20 | × | 99 | + | 20 | × | 10 | x | − | x | ( | 99 | + | 10 | x | ) |
| = | 1980 | + | 200 | x | − | x | ( | 99 | + | 10 | x | ) |
| = | 1980 | + | 200 | x | − | x | × | 99 | − | x | × | 10 | x |
| = | 1980 | + | 101 | x | − | x | × | 10 | x |
| 1980 | + | 101 | x | − | x | × | 10 | x | = | 1980 |
| x2= | 101 10 |