The Gaussian trick allows you to find the sum of every whole number between 1 and some large number n. For example, this could be used if you wanted to find the sum of every number between 1 and 100. You add 1 to the final number and multiply this number by the final number divided by 2:
(100 + 1)*(100/2) = 101*50 = 5,050
A finger math technique allows you to multiply any integer by nine. Start by laying out your hands in front of you. Imagine that the digits 1 to 10 are inscribed on each finger or thumb, running left to right. Fold down the thumb or finger that lies at the digit you wish to multiply by nine. The result will be represented by the number of fingers to the left of this folded digit being the first digit in the result and the number of fingers to the right of this folded digit being the second digit in the result.
One technique for rapidly adding several numbers is to find subsets of numbers that add up to some easily memorized number and then add the remaining numbers to this easily memorized number. For example, if you wished to find 1 + 5 + 4 + 9 + 11, you could start by adding 1 + 5 + 4 = 10. Then add 9 + 11 = 20. Finally, add 20 to 10 to get 30.
The rule of 72 is a rapid math trick that enables you to mentally estimate the effect of compound interest on your bank balance. It enables you to find the amount of time it takes to increase the amount of money in your bank account by a factor of two, given a particular interest rate. You do this by dividing 72 by your percentage interest rate. For example, if you have an interest rate of 8 percent:
72/8 = 9
This indicates that it will take nine years for your money to double if you have an 8 percent annual compound interest rate. The actual value is 9.1 years, so the rule of 72 produces a very accurate estimate.