



 
Hi Amanda. Let b = the speed of the boat relative to still water. This is the constant speed you are trying to find. Then b  4 is its speed going upstream and b + 4 is its speed returning downstream. Let t = the time it takes to get to the far point where the boat turns around. Then 12  t = the time it takes for it to return (because the total time is 12 hours). Now we know that speed = distance / time. So we can write the two legs of the journey as follows:
So we have two equations with two unknown variables t and b. Since we really only care about b, we should solve one of the equations for t and then substitute. Let's use the upstream equation:
Next, substitute this expression for t into the second equation wherever you see t:
Can you solve the equation for b now?  


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