outline:
Let " is divisible by 3" for all .
Clearly is true (Check this!)
Now, assume is true, that is, for some (which means is divisible by 3). we need to show that is divisible by 3.
Now,
...
i leave the last step to you
EDIT: Hey Moo! I thought you were going to bed! you couldn't resist stealing another question from me, could you?