Question 1157569
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Here is an alternative to the standard algebraic solution method shown by the other tutor.<br>
Allen can do the job alone in 5 hours.  So when he and his son complete the job in 3 hours, Allen does 3/5 of the job.<br>
That means his son does 2/5 of the job in 3 hours.<br>
There are two easy paths to the answer from here.<br>
(a) The amount of time it takes the son alone to do the job is 3/(2/5) = 3*(5/2) = 15/2 hours, or 7 1/2 hours.<br>
(b) Since the son does 2/5 if the job in the time Allen does 3/5, the son works 2/3 as fast as Allen; therefore the amount of time it will take the son alone is 3/2 what it takes Allen alone: (3/2)*5 = 15/2 or 7 1/2 hours.<br>