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GNU Emacs Calc 2.02 Manual

3.7.13 Arithmetic Tutorial Exercise 2

27 RET 9 B could give the exact result 3:2, but it does a floating-point calculation instead and produces 1.5.

Calc will find an exact result for a logarithm if the result is an integer or the reciprocal of an integer. But there is no efficient way to search the space of all possible rational numbers for an exact answer, so Calc doesn't try.