| ( | 21 | − | 21 | ) | ÷ | ( | 21 | − | 16 | ) | = | ( | 40 | − | t | ) | ÷ | ( | t | − | 30 | ) |
| 方程两边同时乘以: | ( | 21 | − | 16 | ) | , | ( | t | − | 30 | ) |
| ( | 21 | − | 21 | ) | ( | t | − | 30 | ) | = | ( | 40 | − | t | ) | ( | 21 | − | 16 | ) |
| 21 | ( | t | − | 30 | ) | − | 21 | ( | t | − | 30 | ) | = | ( | 40 | − | t | ) | ( | 21 | − | 16 | ) |
| 21 | ( | t | − | 30 | ) | − | 21 | ( | t | − | 30 | ) | = | 40 | ( | 21 | − | 16 | ) | − | t | ( | 21 | − | 16 | ) |
| 21 | t | − | 21 | × | 30 | − | 21 | ( | t | − | 30 | ) | = | 40 | ( | 21 | − | 16 | ) | − | t | ( | 21 | − | 16 | ) |
| 21 | t | − | 21 | × | 30 | − | 21 | ( | t | − | 30 | ) | = | 40 | × | 21 | − | 40 | × | 16 | − | t | ( | 21 | − | 16 | ) |
| 21 | t | − | 630 | − | 21 | ( | t | − | 30 | ) | = | 840 | − | 640 | − | t | ( | 21 | − | 16 | ) |
| 21 | t | − | 630 | − | 21 | ( | t | − | 30 | ) | = | 200 | − | t | ( | 21 | − | 16 | ) |
| 21 | t | − | 630 | − | 21 | t | + | 21 | × | 30 | = | 200 | − | t | ( | 21 | − | 16 | ) |
| 21 | t | − | 630 | − | 21 | t | + | 21 | × | 30 | = | 200 | − | t | × | 21 | + | t | × | 16 |
| 21 | t | − | 630 | − | 21 | t | + | 630 | = | 200 | − | t | × | 21 | + | t | × | 16 |
| 0 | t | − | 0 | = | 200 | − | 5 | t |
| 5 | t | = | 200 | + | 0 |
| 5 | t | = | 200 |
| t | = | 200 | ÷ | 5 |
| = | 200 | × | 1 5 |
| = | 40 | × | 1 |
| t | = | 40 |