number · Year 2 · AC9M2N01
Counting forwards and backwards to 1000
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.
Sample questions
What number comes next? 16, 17, 18, 19, ?
- The numbers go up by one each time.
- After 19 the ones are full, so the tens go up by one and the ones go back to 0.
- 19 + 1 = 20.
Answer: 20
What number comes next? 26, 27, 28, 29, ?
- The numbers go up by one each time.
- After 29 the ones are full, so the tens go up by one and the ones go back to 0.
- 29 + 1 = 30.
Answer: 30
What number comes next? 76, 77, 78, 79, ?
- The numbers go up by one each time.
- After 79 the ones are full, so the tens go up by one and the ones go back to 0.
- 79 + 1 = 80.
Answer: 80