What is this skill?
This skill is about adding a multiple of ten to a two-digit number, such as 34 + 20 or 57 + 30. The key insight is that adding tens changes only the tens digit: 34 + 20 is 3 tens plus 2 tens, which is 5 tens, with the 4 ones unchanged, so the answer is 54.
First graders work on this with base-ten blocks, hundred charts and mental math. On a hundred chart, adding ten is the same as moving straight down one row, which gives children a strong visual picture of what is happening.
Why it matters
Adding tens is the first step toward adding any two-digit numbers. When children later add 34 + 25, a common mental strategy is to add the tens first (34 + 20 = 54) and then the ones (54 + 5 = 59). Being fast and confident with the first step makes the whole calculation easier.
It also strengthens place value understanding. Children who see why the ones stay the same when tens are added are learning that each digit in a number has its own job, which matters for every written method they will learn.
Adding tens is also a handy everyday skill. Counting money in dimes, adding up scores in tens, or working out what time it will be in 20 minutes all use the same idea, so children get plenty of chances to practice it outside of school.
Common mistakes
- Adding the ten to the ones digit, so 34 + 20 becomes 36 or 34 + 2. Saying "34 plus 2 tens" and showing it with blocks helps children see that the 2 means two tens.
- Changing both digits, for example getting 56 for 34 + 20, by adding 2 to each digit. Remind your child that the number being added has zero ones.
- Counting on by ones from 34 to 54, which is slow and easy to get wrong. Encourage counting on by tens instead: "34, 44, 54."
How to help at home
- Use a hundred chart. Put a coin on 23, then ask your child to add 10 by moving down one row, then another 10, and say the numbers they land on.
- Practice counting by tens from any starting number: 7, 17, 27, 37. It is the same skill and is easy to do out loud anywhere.
- Use dimes and pennies. Start with 3 dimes and 4 pennies, add 2 more dimes, and ask how much money there is now.
- Ask "10 more" and "10 less" questions: "What is 10 more than 46? What is 20 more?" These build the mental habit quickly.
Worked examples
These are real questions from the Tutorsaurus adding tens practice, with the answers worked out.
Example 1
59 + 40 = ?
Answer: 99. Adding 40 only changes the tens: 59 + 40 = 99.
Example 2
58 + 30 = ?
Answer: 88. Adding 30 only changes the tens: 58 + 30 = 88.
Example 3
53 + 40 = ?
Answer: 93. Adding 40 only changes the tens: 53 + 40 = 93.