What is the oxidation number of chlorine in Al(ClO4)3?

Answer 1

You can read about it here.
http://www.chemteam.info/Redox/Redox-Rules.html

Please read the above but here is how you do this one.
All compounds are zero.
Al is in group III (or 13 depending onh the system used) so the oxidation state is 3.
You have 12 O at -2 each for -24. That leaves -24+3 = -21. So 3 Cl atoms must be +21 to balance the -21 there which makes +21/3 or +7 for each Cl.