For the answer to the question above, I'll explain my answer below.
Shawn ALONE = 4 hours
Shawn ALONE in 1 hour = 1/4 of the job
Ellie ALONE in 1 hour = 1/6 of the job
TOGETHER in 1 hour = 1/4 +1/6 = 5/12 of the job
THEY will COMPLETE the job in 12/5 hours
ANSWER 2.4 hours OR 2 hours 24 minutes