A group of tourists went downstream a river and are supposed to return in 3 hours. How far can the tourists travel down the river if the speed of the current is 2 mph, and the speed of the boat in still water is 18 mph?

D=vt

So they will travel downstream and upstream the same distance. Downstream they will travel:

d = (2+18) t1

Upstream they will travel:

u = (18-2) t2

Since d=u we can say:

20t1=16t2

We are told the trip lasts 3 hours so t1+t2=3, t2=3-t1, using this in the equation above we have:

20t1=16 (3-t1)

20t1=48-16t1

36t1=48

t1=4/3 hours, and from earlier we saw that d=20t1 so:

d=20*4/3

d=80/3 miles

d=26 2/3 miles.