CBSE Class 1 · Mathematics
Fun with Numbers (Numbers 21 to 99)
Official NCERT chapter from Joyful Mathematics (English) (book code aejm1). ExamMaster notes are original teaching at CBSE Class 1 depth.
This lesson follows the official chapter “Fun with Numbers (Numbers 21 to 99)” in Joyful Mathematics (English). The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.
- CBSE Class 1
- Easy level
- 8 concepts
1Oranges packed 10 to a box are tens
Ten oranges in a box is one ten. Loose oranges are ones. The box is a house, not a decoration.
Count boxes as tens. Count loose oranges as ones. Do not count a box as “one orange”.
Figure. A full box is one ten, not one orange. Loose oranges sit in the ones house.
How it works
- Count full boxesEach box is 1 ten.
- Count loose orangesThose are ones.
- Read both housesTens, then ones.
221 is two tens and one leftover
21 is two boxes and one loose orange. It is not “2 and 1” as two ones.
The leftover sits in the ones house. The two boxes sit in the tens house.
Figure. 21 is two finished tens and one leftover one. It is not the digits 2 and 1 as two ones.
How it works
- See two tensTwo finished boxes.
- See one leftoverOne loose orange.
- Read 21Twenty-one.
Read 21 by houses
What does 21 name if the 2 sits in the tens house?
- 2 in tens20
- 1 in ones1
- 20 + 121
Pro tip. Say the house before you add.
3Read twenty-one as a tens-and-ones sentence
“Twenty-one” is the spoken form of 2 tens and 1 one. “Two one” is not the school reading.
The spoken word still names houses. It is not a new number.
Figure. “Twenty-one” is the spoken form of 2 tens and 1 one. “Two one” is not the school reading.
How it works
- Name the tens wordTwenty.
- Name the ones wordOne.
- Join themTwenty-one.
4Numbers 21 to 99 keep those two houses
Every two-digit number in this chapter has a tens house and an ones house. 47 and 99 use the same two houses.
A third house (hundreds) waits for a later class.
Figure. Every number from 21 to 99 uses these two houses only. A hundreds house waits for a later class.
How it works
- Point at tensThe left digit.
- Point at onesThe right digit.
- Refuse a hundreds houseNot yet.
547 and 74 swap houses and change value
Swap the digits of 47 and you get 74. The ink is the same. The houses swapped. The number changed.
That is why place decides value. It is not a font change.
Figure. Swap the digits and the houses swap. 4 tens and 7 ones is 47; 7 tens and 4 ones is 74 — a different number.
How it works
- Read 474 tens and 7 ones = 47.
- Swap the digits7 tens and 4 ones.
- Read 74A different number.
Swap houses on 47
Write 47, then swap the digits. What new number do you get, and why is it different?
- 474 tens + 7 ones
- Swap7 tens + 4 ones
- 7470 + 4 = 74
Pro tip. Same digits, new houses, new value.
6The next ten is a new full box
After 29 comes 30: the ones house filled to ten and became a new box. The ones house went back to 0.
30 is three tens, not “3 and 0” as two ones.
Figure. After 29 the ones house fills to ten and becomes a new box. 30 is three tens and zero ones, not “3 and 0” as two ones.
How it works
- See 292 tens and 9 ones.
- Add oneOnes become 10, which is a new ten.
- Read 303 tens, 0 ones.
7Compare from the tens house first
52 and 38: compare tens first (5 > 3), so 52 is more. Do not start at the ones house.
If the tens match, then compare ones. 52 and 57 share 5 tens; 7 ones beats 2 ones.
Figure. Compare tens first. 5 tens beat 3 tens, so 52 is more than 38 even though 8 ones beat 2 ones.
How it works
- Compare tensDifferent? Stop there.
- If tens match, compare onesThe leftover decides.
- Name the largerThe first house that differs.
8A jump of one is the next counting word
From 46 the next counting word is 47. A jump of one does not skip a house. It is the next orange, not a new box unless ones were 9.
Jumping from 46 to 56 is a ten-jump. That is a different jump.
Figure. From 46 the next counting word is 47 — one orange, not a new box. A jump from 46 to 56 would be a ten-jump.
How it works
- Stand on 464 tens and 6 ones.
- Add one orangeOnes become 7.
- Read 47The next counting word.
Which is more, 52 or 38? Compare first at
- The tens house
- The ones house
- The colour of the digits
5 tens beat 3 tens. Ones can wait.
Notes
- The official chapter title is “Fun with Numbers (Numbers 21 to 99)”. Teach the school test for that title, not a contest shortcut.
- If a step needs a later class, stop. The next official chapter will pick it up.
Formulas
- A two-digit number is (tens digit\times 10) + ones digit.
- Compare from the tens house first.
Reference tables
Same digits. Different houses. Different value.
| Number | Tens | Ones | Value |
|---|---|---|---|
| 47 | 4 | 7 | 40 + 7 = 47 |
| 74 | 7 | 4 | 70 + 4 = 74 |
Recap
Hold these pegs from the official chapter “Fun with Numbers (Numbers 21 to 99)”. The wording is ExamMaster’s teaching, not a textbook recap.
- Oranges packed 10 to a box are tens
- Ten oranges in a box is one ten. Loose oranges are ones. The box is a house, not a decoration.
- 21 is two tens and one leftover
- 21 is two boxes and one loose orange. It is not “2 and 1” as two ones.
- Read twenty-one as a tens-and-ones sentence
- “Twenty-one” is the spoken form of 2 tens and 1 one.
- Numbers 21 to 99 keep those two houses
- Every two-digit number in this chapter has a tens house and an ones house.
- 47 and 74 swap houses and change value
- Swap the digits of 47 and you get 74. The ink is the same. The houses swapped. The number changed.
- The next ten is a new full box
- After 29 comes 30: the ones house filled to ten and became a new box.
Practise Fun with Numbers (Numbers 21 to 99)
Reading is free and needs no account. Practice, mocks and progress live in the app.
- A 4-question practice set that ends the chapter
- 1 quick check with worked explanations
- Timed mocks scored with the real marking scheme
- Readiness tracked per topic, kept on your device