Sense this would be known to be a 'prism', we would them do the following key point:

Therefore, we would then multiply to following numbers which would be above.
Question #2.The reason to why we would us 'square units' to answer in a area format would be because by doing so, it would then describe the surface of the area. And by knowing what you are measuring something by would be a whole lot easier.
Volume of a Rectangular Prism = Length * Width * Height
Step 1:
Double your dimensions
5 cm x 2 = 10 cm
3 cm x 2 = 6 cm
4 cm x 2 = 8 cm
Step 2:
Multiply them together
Volume = 10 x 6 x 8
Volume = 60 * 8
Volume = 480 cm^3
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4.6 meters per second was her walking speed