Mateo and Ivy are building towers made of blocks. Mateo is building a tower 9 blocks at a time, and Ivy is building a tower 15 blocks at a time. How many blocks high do both of their towers need to be for them to be the same height?
To find the height of the towers, we need to find the least common multiple of 9 and 15.
Prime factorize 9: 3 * 3
Prime factorize 15: 3 * 5
The least common multiple of 9 and 15 is 3 * 3 * 5 = 45.
Therefore, both towers need to be 45 blocks high. Answer: \boxed{45}.