Grade 1Tens and Ones1.NBT.C.4

Adding Tens

Add a multiple of 10 to a two-digit number, like 34 + 20.

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You can find 10 more. Adding 20 or 30 is just doing that a few times.

Press “Add 20.” Two new ten-sticks join the 3 tens. The 4 ones stay the same.

Key idea

To add tens, add to the tens digit. The ones stay the same: $34 + 20 = 54$.

Why it works

20 is 2 tens. Putting 2 more ten-sticks with 34 gives 5 tens and the same 4 ones.

Worked example: Add 2 tens

What is $34 + 20$?
  1. 34 is 3 tens and 4 ones. 20 is 2 tens.
  2. 3 tens + 2 tens = 5 tens.
  3. $34 + 20 = 54$

Answer: 54

Worked example: Tens first in the sentence

What is $40 + 37$?
  1. 4 tens + 3 tens = 7 tens.
  2. The 7 ones stay the same.
  3. $40 + 37 = 77$

Answer: 77

For grown-ups: Count on by tens from any number together: “23, 33, 43 …” It builds the same idea.

Practice

Pass with 3 right answers, the last 2 in a row. Each right answer earns 1 XP.

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Builds on

Leads to

Sources and further reading

  • Common Core State Standards for Mathematics, Grade 1, 1.NBT.C.4: add a two-digit number and a multiple of 10 using tens and ones
  • Institute of Education Sciences practice guide: Assisting Students Struggling with Mathematics: Intervention in the Elementary Grades (WWC 2021006)
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