Question : 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?
Option 1: 15 days
Option 2: 20 days
Option 3: 12 days
Option 4: 16 days
Correct Answer: 16 days
Solution :
Given,
Bobby and Billy can complete a job in 28 days and 20 days
We know, Total work = Efficiency × time
LCM of 28, 20 = 140 i.e. total work
Efficiency of Bobby = 5 units/day
The efficiency of Billy = 7 units/day
Now,
In 5 days, total work done = (5 + 7) × 5
= 12 × 5
= 60
Remaining work = 140 - 60
= 80
The remaining work will be completed by Bobby in $\frac{80}{5}$ days
= 16 days
Hence, the correct answer is 16 days.
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