ADDING, SUBTRACTING WHOLE NUMBERS EASILY |
Break Them Up then Add or Subtract Them
A Natural Number is a positive WHOLE NUMBER such as 17, 3, 0 or 247.
An fast way to add and subtract them is to break them up into
sums and/or differences that are easy to find.
For example, say we have to add 22 + 17.
If we rewrite it as 20 + 2 + 10 + 7, it's obvious the sum = 39.
This technique works especially well when we subtract.
For example 150 23 can be rewritten 150 20 3 = 130 3 or 127
because 150 20 = 130 and 130 3 = 127.
The trick is to use numbers that sum to 5's, 10's and 0's, then add and/or
subtract them in whatever order makes it easy to find mentally.
We use the associative property, that says we can add
and/or subtract in any order as long as we include all the numbers in the question.
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Examples
a) 28 19 = 20 + 8 10 9 = 9 since 20 10 = 10 and 10 + 8 9 = 9 |
b) 35 + 58 = 30 + 5 + 50 + 5 + 3 = 93 do 30 + 50 + 10 (from the two 5's) + 3 |
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Practice
1) Find these sums by the method above. (your numbers may differ from mine).
a) 17 + 14 | b) 55 + 16 | c) 93 + 18 | d) 84 + 17 |
e) 74 + 25 | f) 55 + 36 | g) 27 + 35 | h) 49 + 25 |
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2) Find these differences by the method above. (your numbers may differ from mine).
a) 60 12 | b) 55 16 | c) 97 18 | d) 84 17 |
e) 74 25 | f) 55 36 | g) 270 35 | h) 49 25 |
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Solutions
1) Find these sums by the method above. (your numbers may differ from the answers).
a) 17 + 14 10 + 7 + 10 + 4 = |
b) 55 + 16 50 + 5 + 10 + 6 = |
c) 93 + 18 90 + 3 + 10 + 8 = |
d) 84 + 17 80 + 4 + 10 + 7 = |
e) 74 + 25 70 + 4 + 20 + 5 = |
f) 55 + 36 50 + 5 + 30 + 6 = |
g) 27 + 35 20 + 7 + 30 + 5 = |
h) 49 + 25 40 + 9 + 20 + 5 = |
2) Find these differences by the method above. (your numbers may differ from the answers).
a) 60 12 60 10 2 = 48 |
b) 55 16 50 + 5 10 6 = |
c) 97 18 90 + 7 10 8 = |
d) 84 17 80 + 4 10 7 = |
e) 74 25 75 1 25 = |
f) 55 36 50 + 5 30 6 = |
g) 270 35 270 30 5 = 235 |
h) 49 25 50 1 25 =
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