What is the largest digit n for which 2345n is divisible by 6?

Your answer would be N = 4 
for a number to be divisible by 6, it must also be divisible by 2 and 3.
For a number to be divisible by 2, it must end in an even number: 0,2,4,6, or 8
For a number to be divisible by 3, the sum of the digits needs to be divisible by 3.

Considering 2345N, and knowing N has to be either 0,2,4,6, or 8 in such a way that when added to the other numbers we get a value divisible by 3 you can see that N must be equal to 4.. 2+3+4+5+4=18  and 18 is divisible by 3