Answer:
it has a lower mass/volume ratio
Step-by-step explanation:
Density is the ratio of mass to volume. An elevator will have the same volume, regardless of the number of people. The mass will increase with the number of people.
An elevator with 50 people in it will have a higher mass/volume ratio than an elevator with 3 people in it. Hence the latter is less dense.
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Additional comment
50 persons would require an industrial elevator of perhaps 10,000 pound load capacity. There might be safety issues with carrying that many people.
