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The Prime Number Theorem is a statement in Number Theory which can't even be stated, let alone proved, without real numbers.

EDIT: How about the statement that $\sum^N\phi(n)$ is asymptotic to $(3/\pi^2)N^2$ (where $\phi(n)$ is Euler's phi-function)?

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The Prime Number Theorem is a statement in Number Theory which can't even be stated, let alone proved, without real numbers.