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A farmer can plow a field in 24 hours. His son, with a larger plow,
can plow it in 12 hours. The farmer asked that they work together
so they could finish before the rain started. It rained 7 hours later.
Had they finished plowing before the rain?
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The farmer's rate of work is of the job per hour.
His son's rate of work is of the job per hour.
Their combined rate of work is the sum
+ = + = = of the job per hour.
In 7 hours, they will complete only of the job - not the entire job.
ANSWER. They will not complete the job in 7 hours - it requires 8 hours.
Solved.