How often do you reach for a calculator on your phone when you need to add or multiply numbers? In this age of digital technology, we’ve simply forgotten how to count quickly, and we panic the moment we need to calculate something without a calculator.
In reality, math isn’t as scary a subject as it seems. Like any science, it has its own interesting secrets and tricks.
ou may have learned some of these mental math methods back in school — or perhaps, by reading them now, you’ll be able to teach your child a few tricks of your own. AdmiGram.com will share with you the simplest secrets of math, so you can easily and quickly calculate numbers in your head.
How to Calculate Quickly: Secrets of Mental Math
Fast squaring
© Nathana Rebouças / Unsplash
This technique will help you quickly square a two-digit number ending in 5. Multiply the first digit by itself plus one, then attach “25” to the end of that result. That’s it!
- 25² = (2 × 3) → 6, then attach 25 → 625
Calculating tips
If you need to leave a 15% tip, there’s a simple way to do it. Calculate 10% (divide the number by 10), then add half of that amount to it, and you get your answer:
- 15% of 25 = (10% of 25) + ((10% of 25) / 2)
- 2.50 + 1.25 = 3.75
Complex multiplication
If you need to multiply large numbers and one of them is even, try regrouping them to make the math easier:
- 32 × 125 is the same as:
- 16 × 250 is the same as:
- 8 × 500 is the same as:
- 4 × 1,000 = 4,000
Multiplication by 11
© Chris Liverani / Unsplash
We all know that multiplying by 10 just means adding a 0 to the number. But did you know there’s a similarly simple trick for multiplying a two-digit number by 11? Here it is:
Take the original number and imagine a gap between its two digits (in this example, we’ll use 52):
- 5_2
Now add the two digits together and write the result in the middle:
- 5_(5+2)_2
Your answer is 572.
If the sum of the two digits is itself a two-digit number, just keep the second digit and carry the 1 over to the first digit:
- 9_(9+9)_9
- (9+1)_8_9
- 10_8_9
Which gives you 1,089. This method always works.
Systematized multiplication rules
- Multiplication by 5: Multiply by 10 and divide by 2.
- Multiplication by 6: Sometimes it’s easier to multiply by 3, then by 2.
- Multiplication by 9: Multiply by 10 and subtract the original number.
- Multiplication by 12: Multiply by 10 and add the original number twice.
- Multiplication by 13: Multiply by 3 and add the original number ten times.
- Multiplication by 14: Multiply by 7, then by 2.
- Multiplication by 15: Multiply by 10 and add the original number five times, as in the previous example.
- Multiplication by 16: If you want, multiply by 2 four times. Or multiply by 8, then by 2.
- Multiplication by 17: Multiply by 7 and add the original number ten times.
- Multiplication by 18: Multiply by 20 and subtract the original number twice.
- Multiplication by 19: Multiply by 20 and subtract the original number.
- Multiplication by 24: Multiply by 8, then by 3.
- Multiplication by 27: Multiply by 30 and subtract the original number three times.
- Multiplication by 45: Multiply by 50 and subtract the original number five times.
- Multiplication by 90: Multiply by 9 and add a zero.
- Multiplication by 98: Multiply by 100 and subtract the original number twice.
- Multiplication by 99: Multiply by 100 and subtract the original number.
image on top: Jim Kelly / Flickr
