Author Topic: Stupid Math Tricks  (Read 1047 times)

Fyndhal

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Stupid Math Tricks
« on: February 11, 2015, 04:07:43 PM »
As someone who has struggled with mild dyscalculia most of my life, yet somehow have ended up doing math for a living, I love learning all the weird tricks that exist to help with things. This thread is for anyone who wants to contribute some odd quirk of mathematics they find fun and interesting.

I'll start with one I learned while working as a Bank Teller at Wells Fargo during college. At the end of the day, we had to balance out our cash drawer. Until it was balanced, we could not go home for the day. We'd add up all the cash in the drawer, then add up all the checks/withdrawal slips/etc. and make sure that the values balance. Easy enough, right? Well, you're frequently going very quickly and it's super easy to reverse two numbers while doing your data entry. For example, entering 81 as 18 or 1,024 as 1,042.

So, you do all the math and...you are out of balance. You have more coming in than you are supposed to or more going out. Oh no! Well, the very first thing you do is divide the difference by 9 -- if it is a whole number, odds are good that you reversed two numbers. Try it yourself. If you reverse two numbers and check the difference between the source and the value entered, it will ALWAYS be divisible by 9.

Remember that when balancing your checkbook!

MunkiLord

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Re: Stupid Math Tricks
« Reply #1 on: February 12, 2015, 05:28:07 PM »
That's so awesome. I'm gonna use this.

Fyndhal

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Re: Stupid Math Tricks
« Reply #2 on: February 12, 2015, 11:28:05 PM »
This one is less useful. In fact, it's pretty frivolous.

  • Pick a number between 1 and 9.
  • Multiply this number by 2.
  • Add 5.
  • Multiply by 50.
  • If you have already had your birthday this year, add 1765. If you haven't had your birthday yet, add 1764.
  • Last step: Subtract the four digit year that you were born.
  • The first digit of the answer is the original number you chose. The remaining two digits represent your age.

Fyndhal

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Re: Stupid Math Tricks
« Reply #3 on: February 23, 2015, 09:51:17 AM »
This one I know but rarely remember when I actually need it.

Multiplying 2-digit numbers by 11:

To multiply any two digit number by 11, say 43 x 11, just sum the digits (4 + 3 = 7) and put it between the 4 and 3 to get the answer: 473. Easy, right?

If the sum is greater than 10, just add 1 to the first digit. For example 49 x 11, 4 + 9 = 13, so stick the 3 between the 4 and 9 (439), and increment the 4 to get 539.

You can use the same trick for 3-digit numbers [2]. For example 417 x 11:
4+1 = 5
1 + 7 = 8
Stick them between the 4 and 7 to get 4587.

Steampunkette

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Re: Stupid Math Tricks
« Reply #4 on: February 23, 2015, 12:29:18 PM »
Instantly multiply by 9:

Hold up both hands in front of you. Count the number you want to multiply by 9 on the fingers on the left hand. Put the last finger you counted down. The fingers left of that number are the 10s place. The fingers to the right are the ones place.

So for 4 by 9 you'd put down the fourth finger from the left, your index finger. Left of the index finger on your left hand are 3 fingers. To the right is your thumb and all five fingers of your right hand.

36. Sadly, it doesn't work past 10.