Have you needed to add or subtract two numbers very quickly, and didn't have a calculator nearby, or felt too tired to compute the sum or difference in your head? There are many techniques that can be used that are especially helpful for those who need a helping hand!
Let's work a couple examples.
What is the solution of 147 - 93? There are a few people that can quickly compute this within a second or two. The easiest way to simplify this calculation is to add 7 to 93, which bumps it to 100. If we add 7 to the one number, we must add 7 to the other number, increasing it from 147 to 154.
Now, what is the difference of 154 + 100? That's right! It is 54! What is 147 - 93? It is also 54, because we have maintained the same relationship between both numbers by adding the same number!
Here's another one to try. What is 83 - 36? You can increase 36 to 40 by adding 4, and also increase 83 by 4, which is 87. The difference is 47.
Many of us refrain from subtracting 2-digit numbers in our head, because we don't have experience, or we prefer to write it on paper where we can see it, but also feel more confident that we won't make a mistake when we can see it in writing.
Practice this tip a few times, and don't forget to show this to a friend or co-worker.
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