Answer: The 12th day
Explanation:
We will list multiples of 4 (for Noah) and 6 (for Aniyah) to find which day they will overlap.
4: 0, 4, 8, 12, 16, 20, 24
6: 0, 6, 12, 18, 24, 30, 36
12 is the least common multiple. This means that on the 12th day, both of them will be at the library. It will be Noah's third time visiting and Aniyah's second time visiting from the first set day.
Please note that there will be other days on which they go on the same day overall (such as the 24th), but the first one will be the 12th day.