Question 1141930
Formula for work problems:
Rate of work * Time spent working = Portion of job completed
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The man's rate of work is 1/4 of the job per hour, so after 1 hour of work:
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1/4 job/hour * 1 hour worked = 1/4 of job completed.
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This means 3/4 of the job remains when the son takes over.
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The son's rate of work is 1/3 of the job per hour.  We also know that he will complete the remaining 3/4 of the job.  We are looking for how long it will take him.
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1/3 job/hour * Hours worked (h) = 3/4
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Put this into an equation and solve for h:
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1/3 * h = 3/4
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Multiply each side of the equation by 3:
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h = 9/4
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So, it will take the son 9/4 of an hour...or 2.25 hours...or 2 hours and 15 minutes...to finish mowing the lawn.