| So first i make fractions of the given numbers |
| Jack does | 1/4 of a lawn in an hour |
| Marilyn does | 1/6 of a lawn in an hour |
| if we go searching for a common numerator in those fractions you find |
| Jack | 3/12 of a lawn in an hour |
| Marilyn | 2/12 of a lawn in an hour |
| so their combined speed is |
| J&M | 5/12 of a lawn in an hour (or you could say the do 5 lawn in 12 hours) |