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.
