Question 132050
If Chuck shovels the driveway in 40 minutes by himself, that means that every minute he does
one-fortieth (1/40) of the job. Therefore, if he shovels for 20 minutes by himself he does 
20*(1/40) = 20/40 = 1/2 the job.
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This means that when Joan starts to shovel, half the driveway is done. So the remaining amount
of work to be done is 1/2 of the driveway. Joan shovels at some unknown rate each minute. Call
that unknown rate J. Meanwhile Chuck continues at his rate of 1/40 per minute. So if we multiply
these two rates by 10 minutes we get the remaining 1/2 of the driveway done. In equation form
this is:
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(1/40)*10 + J*10 = 1/2
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Multiply the entire equation (all terms on both sides) by 40 to get rid of the denominator and
the equation becomes:
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10 + 400*J = 20
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Get rid of the 10 on the left side by subtracting 10 from both sides and the equation is:
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400*J = 10
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Solve for J by dividing both sides by 400 and the equation is then reduced to:
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J = 10/400 = 1/40
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How interesting. Each minute Joan shovels 1/40 of the driveway, the same as Chuck does
each minute. Therefore, if she were working alone it would also take her 40 minutes to 
shovel the entire driveway. 
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Hope this helps you to understand the problem a little better.
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