Final answer:
Saidi made the error of incorrectly comparing numerators when comparing 2/3 and 5/9 using common denominators.
Step-by-step explanation:
Saidi made the error of incorrectly comparing numerators when comparing 2/3 and 5/9 using common denominators. When adding fractions, we can only add the numerators if the fractions share the same denominator. We never add denominators. In this case, 2/3 and 5/9 have different denominators and cannot be added directly.