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January 30, 2015

January 30, 2015

Posted by **fiona** on Thursday, April 1, 2010 at 8:43pm.

- math -
**Reiny**, Thursday, April 1, 2010 at 9:48pmlet the speed of freigh train be x mph

then speed of express is x+20 mph

time for freight to go 300 miles = 300/x

time for express to go 280 = 280/(x+20)

300/x - 280/(x+2) = 2

multiply by x(x+20)

300(x+20) - 280x = 2x(x+20)

300x + 6000 - 280x = 2x^2 + 40x

2x^2 + 20x - 6000 = 0

x^2 + 10x - 3000 = 0

(x+60)(x-50) = 0

since x > 0, x = 50

speed of freight = 50 mph

time for freight to go 300 miles = 6 hours

speed of express = 70 mph

time to go 280 miles = 4 hours

(sure enough the difference between their times is 2 hours.)

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