I'm sorry my explanation doesn't make sense.......
Okay. Here is my edited one:
The largest power of 2 that divides 20! is 2^18 .
The largest power of 3 that divides 20! is 3^8.
We want the largest value of n that n is less than or equal to 8.
Therefore, 8 is our answer.
Does this make sense, jfan?