2/3 = 8/12 (GCD of 3 and 4 is 12. 3 * 4 = 12, 2 * 4 = 8)
1/4 = 3/12 (4 * 3 = 12, 1 * 3 = 3)
8/12 - 3/12 = 5/12
You ate 5/12 of the pie
I'm not sure if this really helps anyone that reads it, but that's how I was taught. You can correct me if I'm wrong.
P.S. I know I'm late for it