| 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) |