Answer:
9 mph
Step-by-step explanation:
-Let x be the speed of the boat upstream.
-Since the time upstream and downstream is equal, we will equate their time functions to solve for x:
-Speed upstream=x-8
-Speed downstream=x+8
#We then equate as follows:
![Time =\frac{Distance}{Speed}\\\\Time_1=Time_2\\\\\\\frac{10}{x-8}=\frac{20}{x+8}\\\\10(x+8)=20(x-8)\\\\10x+80=20x-160\\\\10x=240\\\\x=24\ mph](https://tex.z-dn.net/?f=Time%20%3D%5Cfrac%7BDistance%7D%7BSpeed%7D%5C%5C%5C%5CTime_1%3DTime_2%5C%5C%5C%5C%5C%5C%5Cfrac%7B10%7D%7Bx-8%7D%3D%5Cfrac%7B20%7D%7Bx%2B8%7D%5C%5C%5C%5C10%28x%2B8%29%3D20%28x-8%29%5C%5C%5C%5C10x%2B80%3D20x-160%5C%5C%5C%5C10x%3D240%5C%5C%5C%5Cx%3D24%5C%20mph)
Hence, the speed of the boat in still water is 9 mph
1 hr = 60 min
671,000,000 / 60 = 11183333.33 miles per minute