In The Hitchhiker's Guide to the Galaxy, does the answer the Earth was built for refer to the girl?

You ask about the original radio series, not the books, and the answer is yes.

The second radio series (Episode 12) it is revealed that Zaphod Beeblebrox for planning Earth's destruction in order to avoid the ultimate question becoming known. At the time of the destruction, the disruptive effect of the Golgafrinchan B-ark was not known.

Incidentally, it is Slartybartfast who refers to the Vogon destruction of Earth as a "shocking cock up", not Deep Thought.

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