This method was developed by Jakow Trachtenberg, who developed a set of rules while in prison in Germany with his wife during the second world war. He developed a set of rules that help many people make quick mental calculations. The basic rule can allow you to multiply any number by 11 in your head. Take an original number and then add a space between both digits. Then, add the same two numbers and insert this new digit in between the two original numbers and you have your answer. For example, 52 times 11 become 5 (5+2) 2, and then 572. If the two digits combined add to a double digit, you'll have to add the first digit to the first number. For example, 99 times 11 becomes 9 (9+9) 9, then 9 (1) 89, and finally 1089.
If you need to find the square a 2-digit number ending in 5, there is a very easy trick you can use. Multiply the first digit by itself, add 1, and then put the number 25 afterward. For example, to find the square number of 25, multiply 2 by itself for the number 4, add 1 to make 5, and then insert the number 25 for a final answer of 525.
There is an easy way to figure out how to add 15 percent to any number, which often comes in handy when you need to calculate a tip. First, divide the number by 10 and then add to that number, the number that is half its value. For example, to find the tip for a $25, you divide the total by 10 which gives you 2.5, and then add half of 2.5 which is 1.25, for a total tip of $3.75.
One more trick that is easy to learn are divisions by 5. You can do so with any large number by multiplying the original number by 2 and then move the decimal point by one number towards the left. For example, to find the answer to 7859 divided by 5, first you multiply 7859 by 2, which is 15718, and then you move the decimal one space to the left, which means your final answer is 1571.8.