Study guide: Count to 120 and Add with Place Value
Continue through one hundred and use tens and ones when adding or subtracting.
Ideas to explain in your own words
- After ninety-nine, the count reaches one hundred.
- Adding a one-digit number to a two-digit number can make more than nine ones.
- A multiple of ten adds full tens and no extra ones.
- When both amounts are multiples of ten, you can subtract their counts of tens.
Useful vocabulary
- regroup
- Exchange ten ones for one ten without changing the amount.
- hundred
- Ten full tens or one hundred ones.
Watch your reasoning
- Counting does not restart at one after ninety-nine.
- Do not keep ten ones in the ones column after also recording their new ten.
- Adding full tens does not change the ones count.
- Do not report only the number of tens when the question asks for the whole amount.
A worked review example
Starting at 97, four steps are 98, 99, 100, 101. The last number is 101. Ninety-nine plus one completes one hundred, and the next object gives one hundred one. The starting position is not included among the four new steps. Use a new box for each counting step. Counting does not restart at one after ninety-nine. Write a short consecutive number strip, including 99,100,101 where relevant.
Make a practice plan
- Continue Counting Across One Hundred: Continue 117,118,__ ,__ . Expected: 119,120.
- Exchange Ten Ones When Adding: Find 56+8. Expected: 64, because fourteen ones become one ten and four ones.
- Add a Multiple of Ten: Find 35+40. Expected: seven tens and five ones, or 75.
- Subtract Whole Tens from Whole Tens: Find 90−50. Expected: four tens, or 40.