Numbers are very regular and make beautiful patterns, and if you know those patterns, you can do some pretty cool tricks. Some tricks let you add and multiply big numbers in your head. It’s easy to multiply a big number by 11, and to square numbers ending in 5 (to square a number means to multiply it by itself). As this video on vedic math shows, you can even multiply big numbers by other big numbers in your head. Today we’ll show you some of the coolest number tricks out there, so you can wow your family and friends!
Wee ones: A fun trick is to figure out whether you can cut a number into 3 equal pieces (called dividing). If its digits add up to something divisible by 3, then the original number is divisible, too! Is 141 divisible by 3?
Little kids: To multiply a 2-digit number by 11, you just add its two digits and stick the answer in between the 2 digits. So for example, 32 times 11 is 352 (because 3+2=5). Quick, can you multiply 43 x 11? Bonus: To multiply a big number by 5, you cut it in half and tack on a zero. Quick, what’s 24 x 5?
Big kids: That works in the opposite direction, too: to divide by 5, you chop a zero off and then double what’s left. Quick, what’s 620 divided by 5? Bonus: To get the square of any 2-digit number that ends in 5, you take the first digit, multiply it by the next digit up, and then tack on 25 to the end. So for example, 35×35 is 1225, because 3×4=12, and then you tack on 25. Quick, what’s 55×55?
Wee ones: Yes, because 1+4+1=6, and if you hold up 6 fingers, you see that you can group them into sets of 3.
Little kids: 473. Bonus: 120, because half of 24 is 12.
Big kids: 124, since that’s 62 x 2. A great way to figure out the tip at restaurants. Bonus: 3025, since 5 x 6 is 30, then tack on a 25.
And a big thank-you to Talie B. for sharing that video!