It would take Jack $4$ hours to mow the lawn if he works alone. Fortunately, after he mows for $2$ hours, Jill joins him. They finish mowing $90$ minutes later. How many minutes would it have taken for Jill to mow the entire lawn alone? Enter your answer as a number without units.

12 hours

Step-by-step explanation:

In 1 hour jack mows 1/4 of the lawn

Together they mow half a lawn in 1.5 hours

Which is a whole lawn in 3 hours

in one hour together they mow 1/3 of the lawn

It takes Jill x hours to mow the lawn

in one hour Jill mows 1/x of the lawn

Working together in one hour

jack + jill = together

1/4 + 1/x = 1/3

1/x = 1/3 - 1/4

1/x = 4/12 - 3/12

1/x = 1/12

Cross multiply

12 = x

It takes Jill 12 hours to mow the lawn