SOLUTION: A computer can process a company's payroll checks in 4 hours. A second computer, twice as fast is available. When the smaller computer has worked for 2 hours, the second faster mac

Question 196518: A computer can process a company's payroll checks in 4 hours. A second computer, twice as fast is available. When the smaller computer has worked for 2 hours, the second faster machine is put on line. How much longer did it take the two machines working together to finish the payroll?
My equation: 1/4 + 2(1/4)=1/t/2
I not sure how to represent the fact the first machine has already completed half of the work.
Combined the two computers can complete 3/4 of the work per hour
the first computer worked for 2hours completing 1/2 of work

Answer by scott8148(6628) (Show Source): You can put this solution on YOUR website!
if t equals the time remaining,
then t/4 is the fraction comleted by the first machine
and t/2 is the fraction completed by the second machine

since the first machine has completed half the task, the sum of the fractions is 1/2
t/4 + t/2 = 1/2

alternatively; looking at the whole task
(t+2)/4 is the fraction completed by the 1st machine
and t/2 is the fraction completed by the 2nd machine

(t+2)/4 + t/2 = 1

the two equations are equivalent
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