Lesson 4.2 · Place Value
Tens and ones
Counting lots of things one by one is slow. A faster way is to make groups of ten. Then a number like 34 tells you something neat: 3 groups of ten and 4 more.
Make a ten
Ten ones can be put together to make one ten. In this lesson, a full row of 10 blocks is one ten:
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
A single block is one one: 🟧
10 ones = 1 ten
When you have 10 ones, you can trade them for 1 ten. It is still the same amount!
Two-digit numbers
A two-digit number has two places.
Definition
Tens and ones
In a two-digit number, the left digit tells how many tens. The right digit tells how many ones. In 34, the 3 means 3 tens and the 4 means 4 ones.
The teen numbers are 1 ten and some ones. 16 is 1 ten and 6 ones.
Numbers like 20, 30 and 40 have 0 ones. 40 is 4 tens and 0 ones.
Worked example: Count the blocks
What number is this?
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
🟧🟧🟧🟧🟧
There are 2 tens and 5 ones. Count the tens: 10, 20. Then count on the ones: 21, 22, 23, 24, 25. The number is 25.
Worked example: Break a number apart
How many tens and ones are in 58?
The 5 is in the tens place, so there are 5 tens. The 8 is in the ones place, so there are 8 ones. So .
Worked example: Put it together
What number has 7 tens and 0 ones?
7 tens is 70. Adding 0 ones keeps it 70. The number is 70.
Common mistake
The order of the digits matters. 3 tens and 6 ones is 36, not 63. Write the tens digit first, on the left.
Tip
Up to 120, a three-digit number starts with 1 hundred. 100 is 10 tens, and 110 is 11 tens.
Practice
What number is this?
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
🟧🟧🟧
Type a number.
How many tens are in 47?
Type a number.
How many ones are in 29?
Type a number.
What number is this?
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
🟦🟦🟦🟦🟦🟦🟦🟦🟦🟦
🟧🟧
Type a number.
What number has 6 tens and 4 ones?
Type a number.
What number has 8 tens and 0 ones?
Type a number.
Which one means the same as 1 ten?
Mia has 17 crayons. She puts them in boxes of 10. How many full boxes of 10 can she fill?
Type a number.