rebecca can type a manuscript in 6 hours. if jessa helps her they can type the whole manuscript in 4 hours. How long would it take jessa if she did all the typing alone?

Let Jessa's total hours working alone=J, then you have:

1/6 + 1/J = 1/4 multiply both sides by 12

2 + 12/J = 3 subtract 2 from both sides

12/J = 1 cross multiply

J = 12 hours - what Jessa will take working alone.