Question 723013
When Alice joins Jane, Jane has already completed (1/2)/4 = 1/8 of the homework.

Rate at which both complete homework = (1/3 + 1/4) = 7/12 per hour.

Time needed for both to complete homework = (1 - 1/8)/(7/12) = (12/7)(7/8)

= 3/2 hours