Doing multiplication tables of 6,7,8,9 using your fingers. Really interesting method in this video.

Very good drawing from Instructables illustrating the process.

Here is how it works:

let n1 and n2 be the two numbers between 6 and 10 to multiply.

The number of fingers below the two touching fingers are used to get the 10’s number.

((n1 – 5) + (n2 – 5)) * 10

The fingers below are used to get the units

(10 – n1) * (10 – n2)

Now with some algebra we can see how we will get the product of n1 * n2:

1: ((n1 – 5) + (n2 – 5)) * 10 = (n1 + n2 – 10) * 10 = 10n1 + 10n2 – 100

2: (10 – n1) * (10 – n2) = 100 – 10n1 – 10n2 + n1 * n2

Now we add the two calculations:

10n1 + 10n2 – 100 + 100 – 10n1 – 10n2 + n1 * n2 =

10n1 – 10n1 + 10n2 – 10n2 – 100 + 100 + n1 * n2 =

n1 * n2 √

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