secrets-of-mental-math-malestrom

84
Secrets of Mental Math
439
52196
2000
196
150
46
45
1
1
Answer: 439 5
Actually, there is a much easier way to solve this last problem.
We can simplify our problem by doubling both numbers. Since
2196 2 4392, we have 2196 5 4392 10 439.2 or
439 2
1
.
0
We’ll see more division shortcuts in the next section.
EXERCISE: ONE-DIGIT DIVISION
1. 318 9 2. 726 5 3. 428 7
4. 289 8 5. 1328 3 6. 2782 4
THE RULE OF “THUMB”
When dividing in your head instead of on paper, you may find it
difficult to remember parts of the answer as you continue to calculate.
One option, as you’ve seen, is to say the answer out loud
as you go. But for greater dramatic effect, you may prefer, as I
do, to hold the answer on your fingers and say it all together at
the end. In that case, you may run into problems remembering
digits greater than five if, like most of us, you have only five fingers
on each hand. The solution is to use a special technique,
based on sign language, which I call the Rule of “Thumb.” It is
most effective for remembering three-digit and greater numbers.