Answer
reasonable answers are approximately 2 hours
unreasonable answers are over 2 hours
Work Step by Step
A completes job in 5 hours (1/5 job done in an hour)
B completes job in 3 hours (1/3 job done in an hour)
$1/5 + 1/3 = 3/15 + 5/15 = 8/15$
Combined, the two people complete $8/15$ in an hour. Thus, if we double the time, the two people complete $16/15$ of the job.
