Answer:
5 weeks
Step-by-step explanation:
John creates apps at the rate ...
... (3 apps)/(6 weeks) = 1/2 app/week
Ben creates apps at the rate ...
... (3 apps)/(2 weeks) = 3/2 app/week
Together, they create ...
... (1/2) app/week + (3/2) app/week = 2 app/week
So, the time for 10 apps is ...
... (10 app)/(2 app/week) = 5 week
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Comment on the problem
If the guys actually work at a uniform rate like this, the result after 5 weeks will be 9 completed apps and 2 half-completed apps. Their rate is 10 apps in 5 weeks, but their actual output in that time period may not be 10 completed apps.
One should beware of the "mythical man-month." Rates of job completion depend on many factors. Individual average rates cannot always be summed when folks "work together."
