River Current Speed: 2m/s or 4m/s?

[Krystan]

Homework Statement

Shareen finds that when she drives her motorboat upstream she can travel with a speed of only 8m/s, while she moves with a speed of 12m/s when she heads downstream. What is the current of the river on which Shareen is traveling?

The Attempt at a Solution

x= current speed
b= boat speed

x+b= 12m/s
b-x= 8m/s
b=x+8
x+x+8= 12
2x= 4
x=2

12m/s-8m/s=4m/s

Our physics class is having a debate on whether the answer is 2m/s or 4m/s and why.

 

[krausr79]

I agree with your first answer, for reasons the algebra suggests. Maybe solve for the boat speed in either case? It will likely be inconsistent at 4m/s.