Final answer:
Mario's mistake is in Step 1.
Step-by-step explanation:
Mario's mistake is in Step 1. When subtracting fractions, the denominators must be the same. In Step 1, Mario subtracted 1/3 from 4/3, but he did not make the denominators the same. The correct step should be 4/3 - 1/3 = (4 - 1)/3 = 3/3 = 1. Therefore, the mistake is in Step 1.