13. The greatest among the common divisors of two non-zero natural numbers is called the greatest common divisor (GCD) of these two numbers. If the greatest common divisor of two non-zero natural numbers is 1, then these two numbers are called coprime or relatively prime, for example, 2 and 3 are coprime, 3 and 8 are coprime; 12 and 15 are not coprime, because their greatest common divisor is 3. Among the natural numbers not exceeding 81, there are _ numbers that are coprime with 81.
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