How many hours?
A father and his son can finish digging a well if the father works for 6 hours and his son for 12 hours, or they can finish digging if the father works for 9 hours and his son 8 hours.. It will take for them hours to finish digging if they work together.
If and are coprime positive integers, find .
The answer is 67.
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1 solution
Let x= portion of the father
y= portion of the son 6(1/x)+12(1/y)=1
9(1/x)+8(1/y) =1
solve for x and y we get x=15 and y=20
There fore, the required is 1/x +1/y =1/t where t is the time needed to finish the job if they work together 1/15 +1/20 =1/t t=60/7
Thus 60+7=67