Answer:
a
Step-by-step explanation:
just did this last period
You can express the edge lengths in terms of "cubes" or you divide the total volume by the volume of a cube. It works either way.
Edge lengths are
.. 80 cubes by 8 cutes by 13 cubes
so total volume is
.. (80 * 8 * 13) = 8320 cubes
In cubic inches, the volume is
.. (20 in)*(2 in)*(3 1/4 in) = 130 in^3.
The volume of a 1/4-in cube is (1/4 in)^3 = 1/64 in^3.
Then the number of cubes that will fit in the prism is
.. (130 in^3)/(1/64 in^3) = 8320 . . . . cubes
8320 cubes are needed to fill the rectangular prism.
We can set it up like this, where <em>s </em>is the speed of the canoeist:

To make a common denominator between the fractions, we can multiply the whole equation by s(s-5):
![s(s-5)[\frac{18}{s} + \frac{4}{s-5} = 3] \\ 18(s-5)+4s=3s(s-5) \\ 18s - 90+4s=3 s^{2} -15s](https://tex.z-dn.net/?f=s%28s-5%29%5B%5Cfrac%7B18%7D%7Bs%7D%20%2B%20%5Cfrac%7B4%7D%7Bs-5%7D%20%3D%203%5D%20%5C%5C%2018%28s-5%29%2B4s%3D3s%28s-5%29%20%5C%5C%2018s%20-%2090%2B4s%3D3%20s%5E%7B2%7D%20-15s)
If we rearrange this, we can turn it into a quadratic equation and factor:

Technically, either of these solutions would work when plugged into the original equation, but I would use the second solution because it's a little "neater." We have the speed for the first part of the trip (9 mph); now we just need to subtract 5mph to get the speed for the second part of the trip.

The canoeist's speed on the first part of the trip was 9mph, and their speed on the second part was 4mph.