What is this skill?
Adding within 100 means adding two-digit numbers, such as 34 + 25 or 47 + 38, where the total is no more than 100. Second graders use place value strategies: adding tens and ones separately, counting on by tens and ones, making a ten, and eventually the written column method.
Some problems need regrouping, often called carrying. In 47 + 38, the ones add up to 15, which is more than 9. Fifteen ones are regrouped as one ten and five ones, so the extra ten joins the tens: 4 tens + 3 tens + 1 ten = 8 tens, and the answer is 85.
Why it matters
Two-digit addition is a central skill of second grade and is used throughout later math: adding three-digit numbers, multiplying multi-digit numbers, measurement and money. More importantly, regrouping is where place value understanding gets tested. Children who understand why a ten is carried can work with any size of number later.
Mental strategies matter too. Adding 47 + 38 as 47 + 40 − 2, or 45 + 40, gives children flexible tools for everyday calculations and a way to check written answers.
In second grade, children are expected to understand why the methods work, not just follow steps. A child who can explain that the small 1 written above the tens column is really ten ones that were regrouped has the understanding needed for adding three-digit numbers next, and for subtracting with regrouping, where a ten is broken back into ones.
Common mistakes
- Forgetting to regroup, writing 47 + 38 = 715 or 75. Encourage your child to look at the ones first and ask "Is that more than 9?"
- Adding the carried ten to the wrong place or forgetting it. Writing a small 1 above the tens column, and saying "one ten" rather than "one," helps.
- Misaligning digits when writing problems vertically, so a ones digit ends up in the tens column. Using squared paper or drawing columns helps keep the places lined up.
How to help at home
- Use dimes and pennies to model problems. When there are ten or more pennies, trade ten of them for a dime. This is regrouping, done with real money.
- Practice splitting: "47 is 40 and 7, 38 is 30 and 8. Add the tens: 70. Add the ones: 15. Put them together: 85."
- Estimate first. Ask "About how much will 47 + 38 be? Is it closer to 80 or 100?" Then check the exact answer against the estimate.
- Use real situations: adding scores in a board game, totalling prices of two items under a dollar, or adding pages read on two days.
Worked examples
These are real questions from the Tutorsaurus adding within 100 practice, with the answers worked out.
Example 1
11 + 5 = ?
Answer: 16. 11 + 5 = 16. Add the ones, then add the tens.
Example 2
53 + 18 = ?
Answer: 71. 53 + 18 = 71. Ones: 3 + 8 = 11, so write 1 and carry a ten.
Example 3
51 + 20 = ?
Answer: 71. 51 + 20 = 71. Add the ones, then add the tens.