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To find prime number common divisors of the numerator and  denominator of a fraction. Press the arrow keys to find a prime divisor of the numerator. To select the prime divisor, click on the numbers across the vertical line directly to the right of the numerator. To see if the number is a divisor of the denominator, press arrow key to enter the number below the denominator  and to select click on the numbers across the vertical line directly to the right of the denominator. Each time you find a common divisor continue entering primes in order from left to right. When you can find no more common primes, multiply the found common divisors to get the Greatest Common Divisor (GCD).