Multiplication by five, by six and by seven

All these multiplications-5,6, and 7-make use of the idea of “half” a digit. We put “half” in quotation marks because it is a simplified half. We take half the easy way, by throwing away fractions if there are any. To take “half” of throwing away fractions if there are any. To take “half” of5, we say 2. It is really 2 1/2, but we won’t use the fractions. So “half” of 3 is 1, and “half” of 1 is zero. Of course “half” of 4 is still 2, and so with all even numbers.

This step is to be done instantly. We do not say to ourselves “half” of 4 is 2 or anything like that. We look at 4 and say 2. Try doing that now, on these digits:

2,6,4,5,8,7,2,9,4,3,0,7,6,8,5,9,3,6,1

The odd digits, 1,3,5,7 and 9, have this special feature of dropping the fractions. The even digits, 0.2.4.6 and 8 give the usual result anyway.