If you take 8/12 and simplify it. Divide 4 from both the numerator and denominator. You'll get 2/3. 2/3 is not equal to 5/8 and you can tell by finding a common denominator. 2/3 and 5/8 have a common denominator of 24.
2/3 = 16/24
5/8 = 15/24
15/24 is not equal to 16/24