secrets-of-mental-math-malestrom

Products of a Misspent Youth: Basic Multiplication 31
But you’ll soon see how much easier it is to think from left to
right than from right to left. (For one thing, you can start to say
your answer aloud before you have finished the calculation. That
way you seem to be calculating even faster than you are!)
Let’s tackle our first problem:
42
7
First, multiply 40 7 280. (Note that 40 7 is just like
4 7, with a friendly zero attached.) Next, multiply 2 7 14.
Then add 280 plus 14 (left to right, of course) to arrive at 294,
the correct answer. We illustrate this procedure below.
42
7
40 7 280
2 7 14
294
(40 2)
We have omitted diagramming the mental addition of 280
14, since you learned how to do that computation in the last
chapter. At first you will need to look down at the problem
while doing the calculation. With practice you will be able to
forgo this step and compute the whole thing in your mind.
Let’s try another example:
48 (40 8)
4
Your first step is to break down the problem into small multiplication
tasks that you can perform mentally with ease. Since