SOLUTION: Kristen can finish a task in 5 hours. When Cristina helps Kristen, they can finish the task in 4 hours. How long does it take Cristina to finish the task if she works alone?

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Question 511344: Kristen can finish a task in 5 hours. When Cristina helps Kristen, they can finish the task in 4 hours. How long does it take Cristina to finish the task if she works alone?
Answer by oberobic(2304) About Me  (Show Source):
You can put this solution on YOUR website!
Kristen does 1/5 of the whole job per hour.
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Cristina does the the job at an unknown rate per hr: 1/C
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Working together they can do the whole job in 4 hrs.
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(1/5 + 1/C)*4 = 1 whole job
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1/5 + 1/C = 1/4
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1/C = 1/4 - 1/5
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1/C = 5/20 - 4/20
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1/C = 1/20
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Cristina can do the job working alone in 20 hr.
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Check to see if they can do the job in 4 hr working together.
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(1/5 + 1/20)* 4 = ??
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(4/20 + 1/20) *4 = 5/20 * 4 = 1/4*4 = 1
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Correct.
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Answer: Cristina takes 20 hr working alone.
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Done.