Step One: Since Jack mows 1 lawn in 7 hours, his mowing rate is 1/7 lawns per hour. Since Marilyn mows 1 laws per five hours, her rate is 1/5 lawns per hour.
Step Two: Jack and Marilyn have a combined rate that is 1/7 + 1/5
Step Three: Using t as the time that it takes for the two of them to mow 1 lawn together we do the following
(Jack and Marilyn rate)*time=1 lawn
(1/7 + 1/5)*t = 1
12/35 * t = 1
t = 35/12 hours
∴It takes them 35/12 hours to mow the lawn together