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So what is "mental math"? |
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Performing calculations in your
mind, without using pencil and paper or a calculator. |
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What do you mean? I can't
use a calculator? |
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It's OK. There are many
"tricks" to use! |
Let's
look at each "trick" more carefully. This page includes lessons on
compatible numbers and estimating.
Click on the link at the bottom of the page to get to
second lesson.
Compatible numbers
are pairs of numbers that can be computed easily. Put compatible
numbers together first, then finish the problem.
|
problem |
compatible
numbers (easy to compute) |
rewrite
problem |
solve |
| 90 + 88 + 10 = |
90 + 88 + 10
= |
90 + 10 +
88 = |
188 |
| 325 + 64 + 175
|
325 + 64 + 175
|
325 + 175
+ 64 = |
564 |
|
5 x 84 x 2 = |
5 x 84 x 2
= |
10
x 84 = |
840 |
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25 x 43 x 4 = |
25 x 43 x 4
= |
100
x 43 = |
430 |
Sometimes an
exact number is not needed for an answer. A rounded number
is "close enough". Another reason to do estimation this way is to
see if your real calculated answer is reasonable.
|
problem |
number that can be rounded |
rewrite
problem |
solve |
|
8.99 x 5 |
8.99 rounds to
9 |
9
x 5 |
around 45 |
|
19.75 ÷
4 |
19.75rounds to
20 |
20÷
4 |
5 |
|
983 - 501 |
501 rounds to
500 |
983 -
500 |
483 |
| 501 +
299 + 301 |
501
rounds to 500;
299 rounds to 300;
301 rounds to 300 |
500
+ 300 +
300 |
1100 |
More examples of
estimation problems:
|
Problem |
numbers |
Answer |
| There
are 28 rows planted. Each row contains approximately 39
plants. About how many plants are needed? |
28 rounds to
30, and 39 rounds to
40 |
28 x 39
can be solved: 30 x
40 = 1200 plants |
| Estimate: 4¾ x 5⅛ |
4¾ rounds to
5, and
5⅛ rounds to
5 |
Estimate: 4¾ x 5⅛
5
x 5 = 25 |
| If you
want to double the flour in a recipe that calls for 2¾cups and
you have 5 cups, do you have enough flour? |
2¾ rounds to
3 |
Since double means to
multiply by 2, and
3
x 2 = 6
5 cups is not enough |
Ready for
more "tricks" (second lesson )?
or practice
with these mental math "tricks"
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