hehe
this is really really easy but I think this could help you anyway, please read.
if it's 30% off you most have 70% of the total left. (100-30=70)
(this is the only thing that require brain activity at all)
this is how you are suppose to think (so you don't feel retarded next time).
We know what 70% of the total is, the key is now to find out what 1% is:
70% = 13 <=> 70%/70 = 13/70 <=> 1% = (13/70).
now that you know what 1% is and all you have to do to find 100% is to take 1% a hundred times
1% = (13/70) <=> "we multiply with 100 on both sides"
100% = (13/70)*100 = 18.57....
I hope this will make you remember how to solve these small percentage problems forever