Calculating GCD of 21, 35 and 60

First, we should prime factorize all the numbers:

 

21 = 3x7

35 = 5x7

60 = 3x4x5

 

The GCD is just the product of all the factors that all three of the numbers share. Since there's no common factor for all three of them, the answer is 1. (because the 1 is a common divisor for all real numbers) 

 

:)