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Addition and Subtraction within 100

Adding and subtracting numbers within 100, with and without regrouping, and solving word problems.

Learn it step by step

Addition joins two groups together

When two groups of objects are put together, we add. $24 + 13$ means starting with 24 and joining 13 more, giving $37$ .

Starting at 24 and jumping 13 more lands on 37.

Subtraction takes a group away

When some objects are removed from a group, we subtract. $37 - 13$ means starting with 37 and taking 13 away, leaving $24$ .

Starting at 37 and jumping back 13 lands on 24.

Regrouping when the ones don't fit

When adding, if the ones add up to 10 or more, 10 ones are exchanged for 1 ten. $28 + 15$ : ones are $8 + 5 = 13$ , so 1 ten is regrouped, giving $43$ .

Addition and subtraction undo each other

Addition and subtraction are inverse operations. If $24 + 13 = 37$ , then $37 - 13 = 24$ and $37 - 24 = 13$ . Use this to check an answer.

Word problems: look for join or take-away clues

Words like "altogether", "in total", or "more" often mean add. Words like "left", "remaining", or "fewer" often mean subtract.

Look and learn

There are 35 apples in a basket. 18 more apples are added. How many apples are there altogether?

"Altogether" means add.
$35 + 18$ : ones $5 + 8 = 13$ , regroup 1 ten.
$35 + 18 = 53$ apples.

A shop has 62 pencils. 27 pencils are sold. How many pencils are left?

"Left" means subtract.
$62 - 27$ : 2 ones is not enough, so regroup 1 ten to make 12 ones. $12 - 7 = 5$ , then $5 - 2 = 3$ tens.
$62 - 27 = 35$ pencils.

Check: if

46 + 23 = 69

, what is

69 - 23

Subtraction undoes addition.
$69 - 23 = 46$ .
This matches the original number, so the addition was correct.

Mind map

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