# Mental Math in Real, Economic Decisions

Mathematics has a bad reputation among too many people. People complain about how hard it is, how boring it is, or how irrelevant it is. In reality, mathematics is easy, interesting, exciting, useful, and totally relevant in day-to-day life. In a previous myTake, I talk about the more economical "dollar menu". Now, I talk about mental math in everyday economic decisions.

## Mental Math Tips:

1. When doing your grocery shopping, you see that the prices appear in a pattern. \$9.99. \$5.99. \$0.99. \$4.59. I don't know why that is in America, but I speculate that it is a marketing strategy to make prices seem slightly lower than they actually are. Anyway, if you ever see a \$0.99, then you just round up to the nearest whole number.

2a. When you see a deal like "2 for \$4.99", you calculate the unit price. You round \$4.99 up to \$5.00 and divide by 2, which is \$2.50 for each item.

2b. You should memorize certain numbers in your head. This rote memorization should be no different than how you memorized the times table in primary school.

100 / 2 = 50

100 / 3 = 33.33...

100 / 4 = 25

100 / 5 = 20

100 / 6 = 16.66...

100 / 7 = 14.29...

100 / 8 = 12.5

100 / 9 = 11.11...

100 / 10 = 10

13 x 13 = 169

14 x 14 = 196

15 x 15 = 225

16 x 16 = 256

17 x 17 = 289

18 x 18 = 324

19 x 19 = 361

20 x 20 = 400

2c. When the ad listing says "buy 10 for \$10", that just means you literally have to buy 10 items in order to qualify for the discount. If you don't buy the 10 items, then you have to pay the original value. Unless you really like the item in great quantities, this kind of deal is usually a trick to get you to buy more than you need.

3. When you see a number like 250 and you have to manipulate the number in your head with arithmetic functions, you should simplify the number further into 200 + 50. When the number is something like 3,529, you should simply further into 3,000 + 500 + 20 + 9, which can be simplified further as 3,000 + 500 + 20 + 10, which can be simplified further as 3,000 + 500 + 30. Granted, you can see the original number as 3,530 or 3,000 + 500 + 30 from the get-go. From there, it may be easier to compute arithmetic functions.

4a. Use your ten fingers to help you count. All ten fingers represent the whole set of fingers. 100%. One finger represents 1/10 of the whole. Your fingers can be an aid, if you want to make sure the cashier gets the change right.

4b. Check the items on your receipt at the check-out.

5a. Anything times 10 will have an extra 0 after the original value. The more zeros the multiplier has, the more zeros the product has. Anything times .10 will have the decimal shift one place value to the left. This may be useful, if you are eating at a restaurant and trying to figure out how much tip you must pay the server for the meal.

5b. Know how to write and read in scientific notation. It's shorter than longhand, when dealing with very big numbers.

6. Know the English customary measurement system and universal metric system.

7. Sometimes, visualizing the items being added, subtracted, multiplied, and divided can help with computation. If you see a "2 for \$4.50" deal, then you can break up the \$4.50 into \$4.00 and \$0.50. You divide \$4.00 by 2 and \$0.50 by 2 to get \$2.25. That said, it really helps to strengthen your working memory and hold more information.

8. Chunking can aid your working memory abilities. If you can find any kind of pattern in the seemingly unorganized collection of data, then you should take advantage of that.

9. You need to nourish your brain cells, so they would work properly. Eat more vegetables, raw and cooked. That is brain food. Vegetables tend to be cheaper than fruits, so you can eat a lot more vegetables for the same amount of money than fruits. Drink water, not sugary drinks. If someone invites you to a restaurant and plans to pay for the whole meal, then you should order the salmon. Fish tends to be on the pricey and luxurious end of goods, but at least you'll be serving your brain well.

10. Practice!

Mental Math in Real, Economic Decisions
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