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It takes a computer 8 days to print all of personalized letters for a national sweepstakes. A new computer is purchased that can do the same job in 5 days. How long would it take to do the job with both computers working on it?
Older computer rate = 1/8 job/day
new computer rate = 1/5 job/day
Let together rate = 1/x
rate + rate = together rate
1/8 + 1/5 = 1/x
5x + 8x = 40
13x = 40
x = 40/13 = 3.08 days (time working together)