Numbers from 10 to 20 Chapter Notes | Mathematics Class 1 ICSE PDF Download

Concept of Tens and Ones 

We know numbers from 1 to 9. 

One-digit numbers (1–9) use only the ones place.

Two-digit numbers need two places:

• Tens place (how many groups of ten)
• Ones place (how many single units)

On an abacus you see this as two rods:

• Right rod (O) for ones
• Left rod (T) for tens
• We can put a maximum of 9 beads in each rod.

Numbers from 10–20

Here is how we write numbers after 9:

Ten:

Eleven:

Twelve: 

Thirteen: 

Fourteen:

Fifteen:

Sixteen:

Seventeen:

Eighteen: 

Nineteen: 

Twenty: 

Comparing Numbers 

We can compare any two numbers using the given rules.

1. Two-digit vs. one-digit:
   Any two-digit number (like 12) is always bigger than any one-digit number (like 7).
   Write “12 > 7.”

2. Two two-digit numbers:
   First compare tens: more tens → bigger number.
   20 (2 tens) > 19 (1 ten)
   If tens are equal, compare ones: more ones → bigger
   17 (1 ten + 7) > 15 (1 ten + 5)
   

Before, After, Between 

Using the number line 10 → 20:

• Before: one less (to the left)

  • Before 13 is 12.

• After: one more (to the right)

  • After 15 is 16.

• Between: the number in the middle of two numbers

  • Between 18 and 20 is 19.

Backward Counting

Counting backward is like going down the stairs or counting from the biggest to the smallest. 

Example: 14, 13, 12, 11, 10

Ordering

• In increasing order, we arrange numbers from the smallest to the biggest. 
• Example: 13, 15, 16, 19 are in increasing order. 
• In decreasing order, we arrange numbers from the biggest to the smallest. 
• 18, 14, 12, 10 are in decreasing order.

Key Takeaways

• Numbers 10–20 use two places: tens (groups of ten) and ones (single units).
• On an abacus, the T-rod shows tens and the O-rod shows ones.
• To compare two-digit numbers, check tens first; if equal, compare ones.
• Use a number line to find the number before, after, or between.
• Arrange numbers in ascending (small→big) or descending (big→small) order.
• Keep practising with beads, blocks, and number lines to master 10–20!