Answer:
If they would both work at the same speed they would each take 4 hours to complete the room. So in total they would work 8 hours. But because Carl can finish three hours before Jennifer 3 hours of the eight already got to Jennifer. Then we divide the resting 5 hours by the two and then we have:
Jennifer:
3+2,5=5,5
Carl:
2,5
( if I am right)