Homework Help: Math

Posted by Analica on Thursday, March 15, 2012 at 9:46pm.

A boat travels 45 miles upstream in 5h. The boat travels the same distance downstream in 3hr. What is the rate of the boat in still water? What is the rate of the current? (thank you)

Math - Steve, Friday, March 16, 2012 at 11:50am
still-water speed: s
river speed: r

45 = 5(s-r) = 3(s+r)

5s - 5r = 45
3s + 3r = 45
multiply to match coefficients of r:
15s-15r=135
15s+15r=225

30s = 360
s=12
r=3

check:
45mi/9mph = 5 hr
45mi/15mph = 3 hr

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