30 is Lucas' number
think of a number that can be divided by 3 and 5 without any remainder
for example, if you try, the number 24, 24 is divisible by 3, but it is not divisible by 5. If you try 25, it is divisible by 5, but not by 3.
so the number that you can think of next is 30, which is between 23 and 32, and is a multiple/divisible by 3 and 5