Math Shortcuts

When I was a child I had my own math shortcuts. Using them meant that I didn't "show my work" in math class, which annoyed many of the teachers, and lowered my grades. I had the correct solutions to my math problems, but I was simply using different algorithms, ones which I had a hard time expressing on paper.

In my mind, for example, 97 x 16 became 100 x 16 (1600) minus 3 x 16 (48). This was easier, and thinking this way became almost automatic, so I might just write down 1552 even though I couldn't explain very well how I arrived at the answer. Teachers called that a problem, but I have noticed that many years later such math shortcuts are being sold in seminars and books.

Creating Your Own Math Shortcuts

You can create your own math shortcuts, and the following may give you some ideas on how to do that. Otherwise, you can try any of the shortcuts and algorithms you read about and adopt the ones that seem to work best for you. Our minds work in slightly different ways after all.

Let's look at an example. Suppose you want to multiply 68 x 6. I immediately think "60 x 6 = 360 and 8 x 6 = 48, and 360 + 48 is 408." This is one way to quickly arrive at a solution without pen and paper. On paper it would likely be expressed like this: (60 x 6) + (8 x 6) = 408.

Here is another shortcut: See it as (70 x 6) - (2 x 6). The accompanying "internal dialog" might go like this: "70 x 6 = 420, but that is two "sixes" too many, so take away two sixes (12) and I have 408." There is often more than one way, and you can use whichever math shortcut is easier for you.

By the way, if the problem was 68 x 9, my mind would immediately focus on the 9, because it is close to 10, and multiplying by 10 is easy. 68 x 10 = 680. Then I just have subtract the extra 68 to arrive at the solution of 612. Look for the numbers that are close to 10, 50, 100, 200 or 1000, and you'll find the easier way to do the math, especially if you are trying to do it in your head.

Percentages are trickier to do as mental math, but there are ways. Suppose you want to figure what the 4.6% sales tax will amount to on your $29 book. A quick way to estimate it is to take 10%, or $2.90, cut that in half to arrive at 5%, or $1.45, and then just guess at around $1.35, because you know 4.6% is a little less than 5%. You could also think of 5% as a 20th of the price - if this is easier - and then round that figure down a bit.

Want a more precise solution? 1% of $29 is easy to arrive at (.29), so multiply that by 4 to arrive at $1.16. (You might think of this as (4 x 30) - 4.) Now add .6% to that. For that, think 6 x 29 = 174, and put the decimal in the right place: .174. Add that .18 (rounded up as the store will likely do) to the 1.16 and you have $1.34 in sale's tax, pretty close to our quick estimate. This isn't as difficult as it might seem once you practice a bit.

Many of these simple methods do require a basic understanding of math. For example, you should be able to immediately place the decimal in the right place in the above example. If you understand the basics, you know that .6% has to be less than 1%, or .29, so it can't be 1.74, and .0174 will immediately appear as too much less than 1%.

Another example: if you don't understand that 123 multiplied by 199 is just adding 123 to itself 199 times - that multiplication is just another way to do addition - you will have problems with these math shortcuts. In that case, you may want to simply use the easiest math shortcut of all - a calculator.

Oh and the solution to that last one is 24,477. And yes, I did do that in my head, so let me know if it is incorrect. Here is the mental dialog: 100 x 123 is 12,300 (just add two 0s), so 200 x 123 is 24,600 (just double the 12,300). Subtract the extra 123 from that (which actually goes like this when I slow my brain down to watch: 24,600 - 200 = 24,400, then add back the extra 77 I subtracted) and you have 24,477.

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