SOLUTION: Julia and Robert can finish a piece of work in 15 days. Julia can do the job alone in 20 days. How long would it take Robert to do the task alone?

Question 916524: Julia and Robert can finish a piece of work in 15 days. Julia can do the job alone in 20 days. How long would it take Robert to do the task alone?
Found 3 solutions by stanbon, ewatrrr, richwmiller:
Answer by stanbon(75887)   (Show Source): You can put this solution on YOUR website!
Julia and Robert can finish a piece of work in 15 days. Julia can do the job alone in 20 days. How long would it take Robert to do the task alone?
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together rate = 1/15 job/day
---
Julia rate = 1/20 job/day
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Robert rate = 1/x job/day
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Equation:
rate + rate = together rate
1/x + 1/20 = 1/15
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1/x = 1/15 - 1/20
--------
1/x = (20-15)/(15*20) = 5/(15/20) = 1/60
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x = 60 days (time for Robert to do the job alone)
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Cheers,
Stan H.
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Answer by ewatrrr(24785)   (Show Source): You can put this solution on YOUR website!
x time it takes Robert to do the task alone
Multiplying thru by 60x so as all denominators = 1
3x + 60 = 5x
60 = 2x
30 = x, time it takes Robert to do the task alone
...
And...checking

Answer by richwmiller(17219)   (Show Source): You can put this solution on YOUR website!
rate for A alone 20
rate for A & B together 15
1/a+1/b=1/x
1/20+1/b=1/15
1/b=1/15-1/20
1/b=20/300-15/300
1/b=5/300
1/b=1/60
b=60
rate for B alone 60
check
1/20+1/60=1/15
60/1200+20/1200=1/15
80/1200=1/15
ok
codewa
ewatrrr original mistake was
3x + 60 = 5x
which should have been
3x + 60 = 4x
then the next step is 60=x
ewatrrr own check is wrong
Her check should be

not

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