The 2 digits in the mystery number have a sum of 6. you say this mystery number if you count by 3`s or count by 4`s. What is the mystery number?
Let's count these numbers out.
3..6..9..12..15..18..21..24
4..8..12..16..20..24
From counting by 3's you can see that even though 15, it's two digits adds up to 6, it's not in the list when counting by 4s.
24 is the only one that is in both list.
2+4 = 6
So the answer is 24.