Illustrative Mathematics Grade 1, Unit 5: Adding Within 100
In this unit, first graders add within 100, combining tens with tens and ones with ones and making a new ten when there are enough ones. They subtract multiples of ten, the numbers said when counting by tens, as in 60 - 20, and explain their thinking. It builds on tens and ones from the last unit, and next they measure lengths and count to 120.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
What is my child learning in Unit 5?
- Adding numbers like 46 + 8 and 46 + 20, combining tens and ones and composing a new ten when needed. Learn more: Adding two-digit numbers within 100
- Subtracting multiples of ten, like 60 - 20, and explaining it as 6 tens take away 2 tens. Learn more: Subtracting tens, like 70 - 30
Sample questions to try together
These are real questions from our practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
If you have 2 tens and 4 ones, and then you get 5 more ones, how many do you have in total?
- 30
- 29
- 28
How you can help
- Use bundles of ten straws and loose ones to show 46 + 8. When there are 10 loose straws, bundle them up, and ask what the new number is.
- Add two prices at the store, like a 40-cent item and a 25-cent item, and ask your child to add the tens first.
- Count back by tens on a walk, starting at 60. Ask how many tens you took away to reach 40, and what 60 - 20 is.
Where kids can get stuck
- Adding the 8 to the tens in 46 + 8 and writing 126.
- Answering 48 for 46 + 20, treating the 20 like a 2.
- Writing 4 for 60 - 20, dropping the zero in the answer.
Practice makes perfect
Bobby's Math Club has practice problems that adapt to your child’s level, each with a detailed explanation and an easy-to-follow video, so your child can build confidence in these skills one step at a time.
Words your child will hear
- tens
- Groups of ten. In 34, there are 3 tens.
- ones
- Single things not in a group of ten. In 34, there are 4 ones.
- sum
- The answer when you add, like 7 in 3 + 4 = 7.
- compose a ten
- Put ten ones together to make one ten.
- multiple of 10
- A number you say when counting by tens, like 30 or 70.
- difference
- The answer when you subtract, like 2 in 9 - 7 = 2.
- place value
- What a digit is worth because of its spot, like the 4 in 42 meaning 40.
Want to help your child master adding within 100?
In Bobby's Math Club, your child practices these skills at just the right level, with a hint when they need one, a clear explanation after every question, and kid-friendly videos. Unlimited practice is free.
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Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
Which math standards does Illustrative Mathematics Grade 1 Unit 5 cover?
Illustrative Mathematics Grade 1 Unit 5 covers these math standards. Each one links to what it means, sample questions, and teaching strategies.
For teachers: Unit names and order vary between editions and districts, so your school’s own materials have the exact sequence. Learn how Bobby's Math Club can help your students grow.