Eva and emily can clean the entire house in 4 hours. Eva can do it by herself in 6 hours. How long would it take emily to do it by herself?

Let x = the number of hours Eva needs to complete the job

and y = the number of hours Emily needs to complete the job

x = 6

1/x + 1/y = 1/4

(The amount of work Eva does in 1 hr + the amount of work Emily does in 1 hr should equal 1/4 of the total work)

1/6 + 1/y = 1/4

1/y = 1/12

y = 12 hours

Emily would take 12 hours to do it by herself.