I already knew that the probably that two randomly chosen numbers have a common factor is 6/𝜋^2.

However, I wanted to program this to check for myself.

import math  
import random  

def HCF(a,b):  
  if (a==b) or (a*b == 0):  
    return b  
  else:  
    return HCF(max(a,b)%min(a,b), min(a,b))  


tests = 0  
successes = 0  

while tests < 10000:  
  tests += 1  
  a = random.randint(1,1000000)  
  b = random.randint(1,1000000)  
  if HCF(a,b) ==1:  
    successes +=1  

print("Experimental: ", successes/tests)  
print("Theoretical: ", 6/(math.pi**2))   

Experimental:  0.614  
Theoretical:  0.6079271018540267