The greatest of the common divisors of two non-zero natural numbers is called the greatest common divisor (GCD) of these two numbers. If the GCD 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 GCD is 3. Among the natural numbers not exceeding 81, there are numbers that are coprime with 81.
Problem 770
Official solution
Analysis:
has only the prime factor 3, so all numbers containing the factor 3 are not coprime with 81, , there are 27 numbers not greater than 81 that contain the factor 3, the remaining numbers are coprime with 81, numbers.