| ( | 30 | + | 40 | ) | × | 10 | + | ( | 30 | + | 40 | + | X | ) | × | 4 | = | 120 |
| Left side of the equation = | 30 | × | 10 | + | 40 | × | 10 | + | ( | 30 | + | 40 | + | X | ) | × | 4 |
| = | 300 | + | 400 | + | ( | 30 | + | 40 | + | X | ) | × | 4 |
| = | 700 | + | ( | 30 | + | 40 | + | X | ) | × | 4 |
| = | 700 | + | 30 | × | 4 | + | 40 | × | 4 | + | X | × | 4 |
| = | 700 | + | 120 | + | 160 | + | X | × | 4 |
| = | 980 | + | 4 | X |
| 980 | + | 4 | X | = | 120 |
| 4 | X | = | 120 | − | 980 |
| 4 | X | = | - | 860 |
| X | = | - | 860 | ÷ | 4 |
| = | - | 860 | × | 1 4 |
| = | - | 215 | × | 1 |
| X | = | - | 215 |