Jack usually mows his lawn in 4 hours
In 1 hour, Jack can mow 1/4th of the lawn.
Marilyn can mow the same yard in 3 hours.
In 1 hour, Marilyn can mow 1/3rd of the lawn.
Together, in 1 hr , they both can mow 1/4+1/3 =7/12 of the lawn.
So, to mow the whole lawn, it would take 12/7 hr for both.
12/7 = 1 hr 25 min.