Jack usually mows his lawn in 4hrs Marilyn can mow the same yard in 3hrs. how much time it would take for them to mow the lawn together.
Each hour Jack mows 1/4 of the lawn.
Marilyn mows 1/3 of the lawn.
Together, each hour they mow 1/4+1/3=7/12 of the lawn.
How long would it take to mow the whole lawn:
x/1 = 7/12
Cross multiply to find x.