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 √