Here are some quick tricks to do math fast and easily in your head. To multiply 11 by any two-digit number, say 11 times 43, add the two digits together, then insert the sum between them. In this case, 4 plus 3 equals 7, so the answer would be 473. For numbers with two-digit sums, such as 11 times 84, use a slight variation. Eight plus 4 equals 12, so add the 1 from the 12 to the 8 and leave the two in the middle: 924. Here's another quick trick, one you could use to impress your friends at a party. Ask someone to think of a number. Then have them double it, and then add 12, and then divide by 2. Finally, have them subtract their original number. Before they're finished, tell them the answer: 6. It will always be the same.