Final answer:
The best representation for determining how many planes had an average cruising speed of 0.84 Mach is using the unit 'Mach'.
Step-by-step explanation:
To determine how many planes had an average cruising speed of 0.84 Mach, we can use the representation of the average cruising speed. Mach is a unit of speed representing the ratio of the speed of an object to the speed of sound in the surrounding medium. Therefore, we can say that the best representation for determining this would be using the unit 'Mach' since 0.84 Mach represents the average cruising speed of the planes.
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