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LostInParadise's avatar

Does learning the multiplication table seem less daunting when you realize there are only fifteen entries to memorize?

Asked by LostInParadise (31921points) July 26th, 2019

Here is how to get 15.

The 1 table is transparent.

If you know addition, the 2 table is just adding a number to itself.

For the 5 table, think of dividing by 2 and then multiplying by 10. 5×7 = 7/2×10 = 3.5×10 = 35.

There are two tricks that can be used to multiply by 9. You can multiply by 10 and subtract. 9×7 = 10×7 – 7 = 70 – 7 = 63. Alternatively, use the rule that the first digit is one less than the multiplier and realize the two digits add to 9. So for 9×7, 6 is one less than 7 and 6+3=9.

That leaves 5 numbers – 3, 4, 6, 7 and 8. A 5 by 5 table has 25 entries. 5 of these are along the diagonal – 3×3, 4×4, etc. The remaining ones occur twice – 3×7 and 7×3. It is only necessary to memorize one of these, eliminating (25 – 5)/2 = 10. 25 – 10 =15 entries to memorize. Over the course of a year, that does not seem like all that much.

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