The speed of a stream is 5 mph. A boat travels 2 miles upstream in the same time it takes to travel 12 miles downstream. What is the speed of the boat in still water?
time = distance/speed
speed in still water = v
net speed upstream = v - 5
net speed downstream = 5 + v
So: 2/(v-5) = 12/(5 + v)
Can you take it from here?
[12 + 2] / 2 = 7 mph - speed of the boat in still water.
Check: 7 - 5 = 2 mph - its speed upstream
7 + 5=12 mph - its speed downstream.