In school, you probably had to memorize your times tables through ten or twelve. Now, some schools are requiring your times tables memorized through twenty. However, there is a cool method that can make you multiply these numbers instantly in your head, so quick that it seems like you've memorized them! And you didn't!!
Let's take a simple one, 12 x 13. You may know this one to be 156, or already punched it into your calculator. However, we will try to do it anyways. First, you have to add the last digit of the second number to the first number. 12 + 3 = 15. Next, we tack a zero onto this sum to give us 150. Finally, we multiply the two last digits of your number. 2 x 3 = 6. We add this product onto the number we had earlier (150) to give us 156, exactly what we said.
Let's try a bigger one: 18 x 17. First, you add the 18 + 7, the last digit of the second number, to get 25. Then, we tack on a zero to get 250. Finally, we do 8 x 7 = 56. Add that to 250 and you get the answer of 306, which is the correct answer.
With a little bit of practice, you will be able to multiply these numbers faster than a calculator! Isn't that cool?!
This works well until you come to the number 20. So 20 18 for example is 360. But if you add 20+8 and add a zero you get 280. If you multiply 8x0 you get zero, which makes you short by 80. How do you resolve this?
ReplyDeleteJack Brown
I should have worded it differently, but the simplified version I explained doesn't exactly work for twenty. This is a take on "The Close-Together Method," which is a Vedic system for multiplication. Anyways, the exact formula is not just using the last digit, but using the distance away from the number you multiplied by. Here, it is the distance away from ten because by tacking on a zero (the simplified way to explain it), you are multiplying the number by ten. That means at the end, you must find the product of the distance those two numbers are away from ten and not simply their last digit (which works for every number in the teens). In the case of 20 x 18 as you mentioned, you would multiply 8 x 10 instead of 8 x 0 in order to get the correct answer. There, 280 + 80 = 360, which is the correct answer to 20 x 18. Soon (probably around August), I am going to post on how this works, which will help you understand how to solve that sort of a problem.
ReplyDeleteAmazing method. It really works!
ReplyDeleteFor kids who need to learn the times table I want to share a website
http://mathskillbuilder.org/memorize_multiplication_table.html
It really help memorize the table faster and better than flashcards