| ( | 800 | − | a | ) | ( | 1200 | + | 1 5 | a | ) | = | 854000 |
| Left side of the equation = | 800 | ( | 1200 | + | 1 5 | a | ) | − | a | ( | 1200 | + | 1 5 | a | ) |
| = | 800 | × | 1200 | + | 800 | × | 1 5 | a | − | a | ( | 1200 | + | 1 5 | a | ) |
| = | 960000 | + | 160 | a | − | a | ( | 1200 | + | 1 5 | a | ) |
| = | 960000 | + | 160 | a | − | a | × | 1200 | − | a | × | 1 5 | a |
| = | 960000 | − | 1040 | a | − | a | × | 1 5 | a |
| 960000 | − | 1040 | a | − | a | × | 1 5 | a | = | 854000 |