Grade 1Tens and Ones1.NBT.C.4

Adding Ones to a Two-Digit Number

Add tens to tens and ones to ones.

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Now add a two-digit number and a one-digit number. Add like with like: tens with tens and ones with ones. Your facts do the work.

Press “Add 5.” The new cubes join the ones. The tens do not change.

Key idea

Add ones to ones and tens to tens. $42 + 5$: 2 ones + 5 ones = 7 ones, so the answer is 47.

Why it works

A one-digit number is only ones. It has no tens, so the tens digit stays the same.

Worked example: Add ones

What is $42 + 5$?
  1. Ones: $2 + 5 = 7$.
  2. Tens: still 4.
  3. $42 + 5 = 47$

Answer: 47

Worked example: Small number first

What is $3 + 61$?
  1. Add in any order: $61 + 3$.
  2. Ones: $1 + 3 = 4$. Tens: still 6.
  3. $3 + 61 = 64$

Answer: 64

Common mistake

Adding the one-digit number to the tens, so $42 + 5$ becomes 92. But 5 is 5 ones. The answer should be close to 42, not near 100.
For grown-ups: Ask your child to estimate first: “Will $42 + 5$ be closer to 40 or to 90?”

Practice

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

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Sources and further reading

  • Common Core State Standards for Mathematics, Grade 1, 1.NBT.C.4: add within 100, including a two-digit number and a one-digit number
  • Institute of Education Sciences practice guide: Assisting Students Struggling with Mathematics: Intervention in the Elementary Grades (WWC 2021006)
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