Which fraction is bigger 7/8 or 19/20?

7/8 already reduced to lowest terms, numerator and denominator have no common prime factors, their prime factorization: 
7 is a prime number and 8 = 23;
19/20 already reduced to lowest terms, numerator and denominator have no common prime factors, their prime factorization: 
19 is a prime number and 20 = 2^2 * 5; (and if you solve it up you'll see for yourself that its 19/20)

So 19/20 is bigger.