When you count on by one and the ones digit is 9, the ones are full: they go back to 0 and the tens go up by one. After 109 comes 110. Counting back from a 0, you exchange: one ten becomes ten ones, so before 400 comes 399. Ten more and ten less change the tens digit, and if the tens pass 9 or 0 the hundreds change too.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- count on
- say the numbers that come after
- count back
- say the numbers that come before
- exchange
- swap one ten for ten ones, or one hundred for ten tens
Worked examples
What number comes next? 106, 107, 108, 109, ?
- 1.After 109 the ones are full.
- 2.The tens go up by one and the ones go back to 0: 109 + 1 = 110.
= 110
What number comes just before 400?
- 1.400 has no ones or tens, so exchange one hundred for 9 tens and 10 ones.
- 2.400 − 1 = 399.
= 399
What is 10 more than 395?
- 1.395 has 9 tens. One more ten makes 10 tens, which is one more hundred.
- 2.395 + 10 = 405.
= 405
For parents
Method: Watch the column that fills up
What to say: Which digit changes? What happens when it goes past 9?
Common mistakes:
- Writes '10' inside one column instead of making a new ten or hundred (29 → 210).
- Resets the digit to 0 but forgets to add one to the next column (after 29 comes 20).
- Counting back from a zero, writes 9 but forgets to take one from the next column (before 400 comes 409).
- Changes the wrong column: adds ten when asked for one more, or one when asked for ten more.
- Counts in the wrong direction: gives the number before when asked for after, or more when asked for less.
- Writes a three-digit number the way it sounds, in pieces (one hundred and ten → 10010).