Laura and Brent paddled a canoe 6 miles upstream in four hours. The return trip took three hours. Find the rate at which Laura and Brent paddled the canoe is still water.

Call the rate in still water vs;

upstream you have: vs-v=6/4

downstram you have: vs+v=6/3

where v is the rate with which the river flows.

rearranging:

vs-v=3/2

vs+v=2

from the second you have v=2-vs substitute into the first:

vs-2+vs=3/2

2vs=7/2

vs=7 mi/hr