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Addition Facts up to 100

This lesson covers addition facts up to 100, showing how to break numbers into tens and ones, use doubles and counting-on strategies, and check sums with worked examples so students can add two-digit numbers quickly and accurately.

What Are Addition Facts up to 100?

Addition facts up to 100 are the sums you get when you add two whole numbers and the total stays at or below 100. This includes simple facts like \( 5 + 3 = 8 \), but it also covers two-digit problems such as \( 47 + 36 = 83 \). Knowing these facts well means being able to find the sum quickly, either from memory or by using a reliable strategy, instead of counting one by one every time.

Being comfortable with addition facts up to 100 is an important step after learning smaller facts, because it introduces regrouping (sometimes called "carrying") and prepares students for larger addition and subtraction problems later on.

Using Place Value to Add

The most reliable way to add two-digit numbers is to separate each number into tens and ones, add the matching parts, and then combine the results. For example, to add \( 47 + 36 \):

Break apart: \( 47 = 40 + 7 \) and \( 36 = 30 + 6 \).

Add the tens: \( 40 + 30 = 70 \).

Add the ones: \( 7 + 6 = 13 \).

Combine: \( 70 + 13 = 83 \).

Because the ones added up to more than 10, one extra ten had to be regrouped into the tens column. The diagram below shows this split visually.

Splitting 47 and 36 into tens and ones before adding.

This same idea appears in adding digits up to 100, where numbers are lined up by place value before adding, and it becomes essential once sums require regrouping into a new ten.

Using Doubles Facts

Doubles facts, where both numbers are the same, are some of the easiest addition facts to memorize, such as \( 4 + 4 = 8 \) or \( 9 + 9 = 18 \). Once a doubles fact is known well, it can be used to solve a nearby fact almost instantly. For example, since \( 6 + 6 = 12 \), then \( 6 + 7 \) must be one more, or \( 13 \). This strategy is covered in more depth in adding with doubles up to 100.

Counting On

When one of the numbers being added is small, counting on from the larger number is often faster than breaking numbers apart. To solve \( 58 + 4 \), start at \( 58 \) and count on four more: \( 59, 60, 61, 62 \). This works especially well when the second number is 1, 2, or 3, and it connects directly to the ideas explored in using counting to add digits up to 100.

Worked Example

Find the sum \( 68 + 25 \).

Step 1: Split each number by place value. \( 68 = 60 + 8 \) and \( 25 = 20 + 5 \).

Step 2: Add the tens. \( 60 + 20 = 80 \).

Step 3: Add the ones. \( 8 + 5 = 13 \).

Step 4: Combine, regrouping the extra ten. \( 80 + 13 = 93 \).

So \( 68 + 25 = 93 \). Checking the answer by adding the numbers in the opposite order, \( 25 + 68 \), should give the same total, which is a quick way to confirm the sum is correct.

Practice Tips

Strong recall of addition facts up to 100 comes from repeated, varied practice rather than memorizing every combination separately. Mixing strategies, such as place value splitting for larger sums and counting on for small ones, keeps problem solving flexible. Combining several of these ideas together is also useful once problems ask for the total of more than two numbers, which is explored further in addition strategies.

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