They could mow the lawn in how many hours if they work together

As Jack can mow the yard 7/3 Times faster than Marilyn , Jack can mow 70% of the yard while Marilyn can mow 30% of the yard in the same time Jack would mow 70%.

Jack can mow 10% of the yard in 0.3 hours
Marilyn can mow 10% of the yard in 0.7 hours

Jack will mow 70% of the yard : 0.3 hours ( 10% ) × 7
= 2.1 hours . Jack can mow 70% of the yard in 2.1 hours

Marilyn will mow 30% of the yard : 0.7 hours ( 10%) × 3 = 2.1 hours. Marilyn can mow 30% of the yard in 2.1 hours.

John mowwing for 2.1 hours + Marilyn mowwing for 2.1 hours = 100% of the yard in 4.2 hours.

Awnser : 4.2 hours