How to Easily do Subtraction in your Head
61Children are given calculators at such a young age they have lost the art of mental arithmetic. The brain is capable of so much more than we ask it, especially in the area of mathematics and mental arithmetic. The method of teaching addition and subtraction in today’s schools is one using pencil and paper so when asked to calculate without them, the child is stuck. Here’s a method of subtracting that you can easily do in your head, it works well for 3 digit numbers and some children will be able to use it to subtract 4 digit numbers.
Suppose you want to subtract 8 from 15. Find the difference between 8 and 10 (which is 2), then find the difference between 10 and 15 (which is 5). Then add 5 and 2 to get the answer 7.
Suppose you want to subtract 34 from 77. Find the difference between 34 and 40 (which is 6), then find the difference between 40 and 70 (which is 30), then find the difference between 70 and 7 (which is 7). Then add 6, 30 and 7 to get the answer 43.
Suppose you want to subtract 47 from 521. Find the difference between 47 and 100 (which is 53), then find the difference between 100 and 500 (which is 400) then find the difference between 500 and 521 (which is 21). Add together 53, 400 and 21 to get 474.
Suppose you want to subtract 134 from 974. Find the difference between 134 and 200 (which is 66), then find the difference between 200 and 900 (which is 700), then find the difference between 900 and 974 (which is 74). Then add 66, 700 and 74 to get 840. If finding the difference between 134 and 200 is too hard, you can go via 140. The difference between 134 and 140 is 6, the difference between 140 and 200 is 60, add together 6 and 60 to get 66.
As you can see from the last example, you can break the question into as many steps as you wish to turn a hard subtraction sum into an easy addition sum.
As with all things practice makes perfect, and then you can challenge your friends to give you two numbers and you can find the difference in your head before they can even find a calculator!
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You got the wrong answer for 521 minus 47.... typo I suppose.
Good suggestions. I often break the numbers down as you indicated, which makes it easier to visualize and/or work the math problem.
Neat method. Thanks for the tip!
Please read some of my other hubs
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TinaAtHome Hub Author 23 months ago
Thanks JesusDidThis, I've corrected the error.