How to Multiply up to 20X20 Quickly

Step-by-Step Guide

1. 1

   Step 1: Write or visualize the two numbers in the vertical algorithm.

   This means to stack the numbers on top of each other, as you normally would when multiplying.
   
   Note that this method only works for products through 19×19{\displaystyle 19\times 19}.
   
   For example, if you are multiplying 18×16{\displaystyle 18\times 16}, you would write or think of the numbers with the ones digits lined up vertically (the 8 and the 6), and the tens digits lined up vertically (the two 1s).
   
   It doesn’t matter which number is on top.
2. 2

   Step 2: Draw a circle around the top number and the bottom ones digit.

   You might visualize this as a shape that looks like the African continent.
   
   The only digit that should not be inside the circle is the bottom tens digits.For example, if you are multiplying 18×16{\displaystyle 18\times 16}, you would draw a circle encompassing 18 and
   6. , You should be adding the top number and the bottom ones digit.
   
   Make sure you add rather than multiply.
   
   For example, 18+6=24{\displaystyle 18+6=24}. , You can do this easily by adding a 0 to the end of the number.
   
   Set this number aside, as it is your first partial product.
   
   For example, 24×10=240{\displaystyle 24\times 10=240}. , Make sure you multiply this time, instead of adding.
   
   This is the second partial product.
   
   For example, 8×6=48{\displaystyle 8\times 6=48}. , This will give you the product of your two original factors.For example, 240+48=288{\displaystyle 240+48=288}.
   
   Thus, 18×16=288{\displaystyle 18\times 16=288}.
3. 3

   Step 3: Add the two numbers together.

4. 4

   Step 4: Multiply the sum by 10.

5. 5

   Step 5: Multiply the two ones digits in the original factors.

6. 6

   Step 6: Add the two partial products together.

Detailed Guide

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