Speed Math for Kids Achieve Their Full Potential

Our problem now looks   like    this:

Let’s   check   the answer  by  casting out the nines:

Zero    times   7   is  0,  so  the answer  is  correct.

Using 50 as a reference number

That takes care of the numbers up to around 30 times 30. What if the numbers are higher? Then we can
use 50 as a reference number. It is easy to multiply by 50 because 50 is half of 100. You could say 50 is
100 divided by 2. So, to multiply by 50, we multiply the number by 100 and then divide the answer by
2.
Let’s try it:

Subtract    diagonally:

45  −   3   =   42

Multiply    42  by  100,    then    divide  by  2:

42  ×   100 =   4,200

4,200   ÷   2   =   2,100

Now multiply    the numbers in  the circles,    and add this    result  to  2,100:

3   ×   5   =   15

2,100   +   15  =   2,115

Fantastic.  That    was so  easy.   Let’s   try another:

Add diagonally, then multiply the result by the reference number (multiply by 100 and then divide by
2):

57  +   3   =   60

60  ×   100 =   6,000