Math
posted by Analica .
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)

stillwater speed: s
river speed: r
45 = 5(sr) = 3(s+r)
5s  5r = 45
3s + 3r = 45
multiply to match coefficients of r:
15s15r=135
15s+15r=225
30s = 360
s=12
r=3
check:
45mi/9mph = 5 hr
45mi/15mph = 3 hr