Formula for 11 times table
It is strange, but these two days, Elkan has been saying lots of “I’m so smart!” to himself and no one in particular. I asked him whether he is doing it because he is stressed, but he said that he does it quite often (so I probably was not observant enough).
Actually as early as when he was in nursery, he used to praise himself when he was doing his work. (“Clever boy, Elkan!”, he would say. ) It was strange, but for a 3 year old boy, we thought it was very cute.
Anyway, yesterday he wrote this out, and was very proud of himself. (“Look mum, I’m so smart!”)
I thought I should write it out here, because I may lose the piece of paper he wrote it on…
Formula for 11 times table for 10 to 90
There are two formulas for two types of situation. Example A is simpler, and to be used if the sum of the two digits is less than 10. Example B involves more steps and is to be used if the sum of the two digits is more than 10 (See examples to understand).
Example A : 43 x 11
Step 1 : 4 + 3 = 7 (less than 10)
Step 2 : Stuff the 7 between the 2 numbers.
Answer : 473
Example B : 87 x 11 (this method has to be used if the answer for Step 1 is 10 or more)
Step 1 : 8 + 7 = 15 (more than 10)
Step 2 : 15 – 10 = 5 (i.e. you take the sum from Step 1, and minus 10 from it. “10 is the magic number” our little mathematician says)
Step 3 : 8 + 1 = 9 (i.e. you take the first number and add a “1” (another magic number I suppose..)).
Step 4 : Put 9, 5, and 7 in order (i.e. the answer from Step 3, followed by answer from Step 2, followed by the second digit)
Answer : 957
I don’t see how Example B is a short cut – it seemed rather complicated to me. Maybe that is because I don’t understand Maths. Elkan certainly had fun figuring it out.
