SOLUTION: An old computer takes 15 minutes longer to print a document than the new computer. Together the computers can print the document in 4 minutes. How long for the new computer working

Algebra ->  Rate-of-work-word-problems -> SOLUTION: An old computer takes 15 minutes longer to print a document than the new computer. Together the computers can print the document in 4 minutes. How long for the new computer working      Log On


   

Question 1044142: An old computer takes 15 minutes longer to print a document than the new computer. Together the computers can print the document in 4 minutes. How long for the new computer working alone?
Found 2 solutions by josgarithmetic, advanced_Learner:
Answer by josgarithmetic(39613) About Me  (Show Source):
You can put this solution on YOUR website!
The job of 1 document, shared between two computers? The application is impractical. As just a RATES exercise, 1 whole job is "print the document". 
                   RATES, Jobs per minute

Slow Computer     1%2F%28x%2B15%29

Fast Computer     1%2Fx

Together           1%2F4

The variable assigned to time for the fast or new computer to do 1 job is x.
1%2F%28x%2B15%29%2B1%2Fx=1%2F4
Solve this for x.

Answer by advanced_Learner(501) About Me  (Show Source):
You can put this solution on YOUR website!
answer is 5
this is a quadratic equation to be solved
new computer time t
old computer time is t+15
rate1.time1+rate2.time2=1
%281%2Ft%29%284%29+%281%2F%28t%2B15%29%29%284%29=1
%281%2Ft%29%284%29+%281%2F%28t%2B15%29%29%284%29=1
%284%2Ft%29+%284%2F%28t%2B15%29%29=1
%284t%2B60%2B4t%29%2F%28t%29%28t%2B15%29=1
%288t%2B60%29%2F%28t%29%28t%2B15%29=1
%28t%29%28t%2B15%29=%288t%2B60%29
%28t%29%28t%2B15%29=%288t%2B60%29
%28t%5E2%2B7t-60%29=0
factoring
%28%28t%2B12%29%28t-5%29%29=0
means
t=-%2812%29 negative time is meaningless
t=5
check
new system t=5,
old system t=20