One possible counterexample is:
1/4 + 1/5 + 1/20 = 1/2.
In this problem, we would find the least common denominator for 4, 5 and 20. The first thing all 3 numbers will divide into is 20:
1/4 = 5/20; 1/5 = 4/20; 1/20 = 1/20
This gives us: 5/20 + 4/20 + 1/20
Adding the numerators, we get (5+4+1)/20 = 10/20 = 1/2.
