Math destroyer question...

[drill-and-fill]

---

Members don't see this ad.

The question states that the speed of a moving boat is 8 ft/s while upstream is 4ft/s, and then asks for the speed of the boat in still water. I thought it would be four since thats the difference in speed between up and downstream so that must be the resistance or speed of the river flowing downstream. But the answer is 6. Can someone explain to why? 🙁

 

[ikecarus]

One thing you need to understand is that the boat has a "base" speed and the current has its own speed as well. When the boat is going downstream it is the boat's base speed + the downstream current and when the boat is going upstream it is the boat's base speed + the upstream current. You can assume that the upstream and downstream current are equal.

I set up a system of equations for this but hopefully someone else can explain it better...

Let x = the current and y = the base speed of the boat (aka the boat in still water):

When the boat is going downstream your equation is: y + x = 8
When the boat is going upstream your equation is: y - x = 4

So to solve this system, add the two equations together to get: 2y = 12.
y = 6, the correct answer.

Hope that helps! 🙂