Passage Summary: There are twice as many apartments as there are houses on a street, and all those apartments are inside those houses, so the author assumes most of the houses must be multi-unit buildings.

Conclusion: Most of the old houses on 20th Avenue contain more than one apartment.

Reasoning: Every apartment on 20th Avenue is located in an old house, and there are twice as many total apartments as there are old houses on that street.

Analysis: This argument suffers from a classic numerical flaw regarding averages versus distributions. While the average is two apartments per house, this doesn't mean the majority of houses have multiple units. It is entirely possible that one massive apartment complex contains nearly all the units, while the rest of the houses are single-family homes. Look for an answer that points out that a high total number of apartments could be concentrated in just a few buildings.