Bobby and Billy can complete a job in 28 days and 20 days, respectively. They work together for 5 days, after which Billy leaves the job. In how many days did Bobby alone complete the rest of the job?

16 days

Bobby = 28 days, Billy = 20 days,

⇒ They worked for 5 days together = (5 + 7) x 5 = 12 x 5 = 60 units   ..(Efficiency × Days = Total work)

⇒ Remaining work = 140 - 60 = 80 units,

⇒ As Billy left, So time required by Bobby to complete the remaining work = \(\frac{80}{5}\) = 16 days,     ..(\(\frac{Work}{Efficiency}\) = Time)