For example, it's fairly easy (and I'm about to teach you, so skip if you don't like math) to multiply any 2-digit number by 11. You merely and together the 2 digits, and insert that number between them.
For example, in my head right now, 54x11=594, because 5+4=9, and I just but that between the 5 and 4. Checking with a calculator, it's right.
For digits that add to 10 or more, say, the number 74x11, you merely add a 1 the the one's digit (in this case, 7), and insert the second digit in between. So 7+4=11, 1+7=8, so the answer is 814. Again, checking with a calculator, and it's right.
So there, you just learned a simple but powerful "mathmagic" trick! With practice, you can go really fast.
Like, 99x11=1089 (undoubtedly you'll be asked that one if you brag about your skill).
PS: No idea if this could be used in an actual magic effect (mentalism maybe?) or not, but someone could investigate...