Question 587628
Jerry works 3 hours faster than David. Together they complete 5 jobs in 18 hours. How long does it take Jerry alone? 
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First, I will assume the work rates for David and Jerry are for one job.
let x=hours for Jerry to complete one job alone
5x=hours for Jerry to complete 5 jobs alone
1/5x=work rate for Jerry to complete 5 jobs
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(x+3) hours for David to complete one job alone
5(x+3) hours for David to complete 5 jobs alone
1/5(x+3)=work rate for David
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It took18 hours to complete 5 jobs (100%)
18/5x+18/5(x+3)=1
LCD:5x*5(x+3)
18*5(x+3)+18*5x=5x*5(x+3)
90x+270+90x=25x^2+75x
25x^2-105x-270=0
divide by 5
5x^2-21x-54=0
(5x+9)(x-6)=0
5x+9=0
x=-9/5 (reject, x>0)
x-6=0
x=6
Ans:
hours for Jerry to complete one job alone=6 hrs
hours for Jerry to complete 5 jobs alone=5x=30 hrs
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Note:your set up correctly found the hours for Jerry to complete one job alone, but I see the problem better the way I set it up.