| S | × | 1 | ÷ | 18 | + | S | × | 1 | ÷ | 21 | + | S | × | 2 | ÷ | 21 | = | 3 | ÷ | 60 |
| Left side of the equation = | S | × | 1 18 | + | S | × | 1 21 | + | S | × | 2 21 |
| = | 25 126 | S |
25 126 | S | = | 3 | ÷ | 60 |
| Right side of the equation = | 1 20 |
25 126 | S | = | 1 20 |
| S | = | 1 20 | ÷ | 25 126 |
| = | 1 20 | × | 126 25 |
| = | 1 10 | × | 63 25 |
| S | = | 63 250 |
| S | = | 0.252 |