Math Tricks every Adult should Know

ath is so, so, SO HARD! Especially when you are an adult, since your brain cells have aged. Not like a fine wine, not like cheese either (hard and sharp). Then? Umm...like milk, maybe? Please don’t hurt me, I was kidding! Kinda maybe… Well, in any case, you will definitely need some help with math, like learning (read: re-learning) basic math skills. And tricks and hacks. Well, here they are: some great tips for getting math done.

These eight mathematical tricks can’t be explained easily, but they manage to do the task at hand by putting in half the hard work.

“Magical” Mathematical Tricks Every Adult Should Know

1. Faster Percentage Calculation

Want to know how to calculate a percentage of a number in an easier way? All you have to do is multiply the numbers. Subsequently, you can figure out where the decimal point goes. Just so you remember: a "percent" of something means a fraction out of 100, like 50% means 50 out of 100 or 50/100. This means you move the decimal point two digits to the left to convert it into a percentage.

For example:

What’s 20% of 70? Multiply 20 with 70 - 20 times 70 equals to 1400. The decimal point currently is after the ones place, but if you move it two digits to the left, it is 14.00, so the answer is 14. Also, 70% of 20 also comes to 14.

2. Easy Rules for Divisibility

Picture this: It’s Thanksgiving, and you need to decide quickly if 108 slices of pumpkin pie can be evenly split by 12 people, so here are some helpful shortcuts to figure out the divisibility of numbers. (Note: these shortcuts work for all numbers that do not have fractions and decimals.) A number is:

• Divisible by 2 if the number's last digit is divisible by 2 (like 98).
• Divisible by 3 if the sum of the digits of the number are divisible by 3 (660 is divisible because 6 + 6 + 0 equals 12, which is divisible by 3).
• Divisible by 4 if the last two digits of the number are divisible by 4 (420 is divisible because 20 is a multiple of 4).
• Divisible by 5 if the last digit is 0 or 5 (like 1,395).
• Divisible by 6 if the rules of divisibility for 2 and 3 work for that number (like 318, as the last digit 8 is divisible by 2, and 3 + 1 + 8 equals to 12, which is divisible by 3).
• Divisible by 9 if the sum of digits of the number are divisible by 9 (like 9,252, because 9 + 2 + 5 + 2 equals 18, which is divisible by 9).
• Divisible by 12 if the rules of divisibility for 3 and 4 work for that number (like 912, as 9 + 1 + 2 equals to 12, which is divisible by 3, and 12 is a multiple of 4).
  

3. Faster Square Roots

Everybody knows that the square root of 9 is 3, but do you know the square root of 89 off the top of your head? No? Then here’s how you can give a quick estimate: Find the nearest square root. In this case, the square root of 81 is 9. Then, find the next nearest square root. In this case, the square root of 100 is 10. Therefore, the square root of 89 is between 9 and 10. The number 89 is closer to 81, so the square root will be 9 point something.

4. The Rule of 72

How long will it take for your money to double at a certain interest rate? You can just use “The Rule of 72” to approximate the outcomes of compounded interest.

All you have to do is divide the number 72 by the certain interest rate, and you will get the estimated number of years that it will take for your money to double.

For example:

If you were to invest in a 0.9% CD, it would take about 80 years for your money to double, since 72 divided by 0.9 equals to 80. On the other hand, if you were to invest in a mutual fund with a 7% return, it would take your original funds about 10.29 years to double, since 72 divided by 7 equals to 10.2857, which rounded to the nearest hundredth is 10.29.

5. The Rule of 115

If you are feeling a bit adventurous and want to try and triple the amount of money you have, then substitute the number 115 instead of the number 72 to estimate the number of years it will take your money to triple. For example, an investment at a 4.8% growth rate would take about 23.96 years to triple, since 115 divided by 4.8 equals to 23.958, which rounded to the nearest hundredth is 23.96.

6. Figuring out the Hourly Rate

Picture it: you are looking for a job and you find that you fulfil the requirements for two jobs. If you are able to work the same amount of hours, which job will pay better: one with an annual salary of $42,000 or one with an hourly rate of $39?

You can figure out the hourly rate of the annual salary by dropping the three zeros and dividing that number by 2. In this case, the hourly rate would be 42 divided by 2, which would equal $21 per hour. Keeping all other things equal, the $39 per hour gig will pay better, and will be the better option to choose.

7. Advanced Finger Math

BREAKING NEWS: Your fingers can do more just adding and subtracting numbers. If you have problems remembering the multiplication table of the number nine, then try this finger math trick:

Step One: Open both of your hands and make sure you stretch your fingers in front of you so you can see them clearly separated.

Step Two: For example, if you want to multiply 9 by 3, fold down your third finger from the left (your middle finger on your left hand). If you want to multiply 9 by 4, fold down your fourth finger from the left (your index finger on your left hand), and so on and so forth.

Step Three: The answers to 9 times 3 and 9 times 4 can be found by counting your fingers on either side of the bent finger and combining them: 2 and 7 makes 27 and 3 and 6 makes 36.

Now, you have the multiplication table of 9 at your fingertips up until 9 times 10! (Pun wasn’t intended, but since it happened, it’s okay I guess.)

8. Fast Multiplication by 4

Here’s how you can multiply any number times four at lightning speeds: First double the number and then double it again. Let's use this shortcut with 243 times 4: if you double 243, you get 486 and if you double 486, you get 972, so 243 times 4 is 972.

Now here are some more “practical” tricks for you to use to improve your math skills.

“Practical” Mathematical Tricks That Make Adulting WAY Easier

A Division Trick

This trick is directly related to the “Easy Rules for Divisibility” trick described above. This trick can quickly let you know when a number can be evenly divided by these certain numbers. The number can be divided by:

• 2 if the number ends in 0, 2, 4, 6, or 8
• 3 if the digits of the number are added together and the result is evenly divisible by the number 3
• 4 if the number ends in 00 or a two digit number that is evenly divisible by 4
• 5 if the number ends in a 0 or 5
• 6 if the number is an even number and when the digits of the number are added together the answer is evenly divisible by 3
• 8 if the last three digits of the number are evenly divisible by 8 or are 000
• 9 when the digits of the number are added together and the total is evenly divisible by 9
• 10 if the number ends in 0
  

Tough multiplication

When you are multiplying large numbers, and if one of the numbers is even, you should divide the first number in half, and then double the second number. This method will solve the problem quickly. For instance, consider the equation 30 times 140:

Step One: Divide the 30 by 2, which equals to 15. Double the number 140, which equals to 280.

Step Two: Then multiply your two answers together - 15 times 280 equals to 4200

The answer to 30 times 140 is 4,200.

Now you have all of the tools in your arsenal to take on any math problem in the world! OK, maybe not the world, but at least you won’t be stumped when your boss asks you about data reports or you’re going out with co-workers and you have to split a bill. Right?