Multiplication Tips


Concept:


How To Use the Concept:
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When I went to school, they didn't teach this trick. I think every 8 or 9 year old should know it. If you are a teacher, please pass it on, if you don't teach it already.

I will assume that you know your multiplication table reasonably well up to 10x10. This trick will extend you up to 20x20 in about FIVE minutes of work! You will be able to multiply any two numbers from 11 to 19 in your head quickly and without the use of a calculator.

Try this example:

  • Take 15 x 13 for an example.
  • Always place the larger number of the two on top in your mind.
  • Then draw the shape of Africa mentally so it covers the 15 and the 3 from the 13 below. Those covered numbers are all you need.
  • First add 15 + 3 = 18 add a zero behind it (multiply by 10) to get 180.
  • Multiply the covered lower 3 x the single digit above it the "5" (3x5= 15)
  • Add 180 + 15 = 195.

That is It! Wasn't that easy? Practice it on paper first! Just to be totally honest: 20x20=400

The 11 Rule

 You likely all know the 10 rule (to multiply by 10, just add a 0 behind the number) but do you know the 11 rule? It is as easy! You should be able to do this one in you head for any two digit number.

To multiply any two digit number by 11:

  • For this example we will use 54.
  • Separate the two digits in you mind (5__4).
  • Notice the hole between them!
  • Add the 5 and the 4 together (5+4=9)
  • Put the resulting 9 in the hole 594. That's it! 11 x 54=594

The only thing tricky to remember is that if the result of the addition is greater than 9, you only put the "ones" digit in the hole and carry the "tens" digit from the addition. For example 11 x 57 ... 5__7 ... 5+7=12 ... put the 2 in the hole and add the 1 from the 12 to the 5 in to get 6 for a result of 627 ... 11 x 57 = 627

Square a 2 Digit Number Ending in 5

 For this example we will use 25
  • Take the "tens" part of the number (the 2 and add 1)=3
  • Multiply the original "tens" part of the number by the new number (2x3)
  • Take the result (2x3=6) and put 25 behind it. Result the answer 625.

    Try a few more 75 squared ... = 7x8=56 ... put 25 behind it 5625.
    55 squared = 5x6=30 ... put 25 behind ... is 3025.