Subtract Within 20 Notes & Questions | Class 1 Mathematics | Drishti OS

Subtract Within 20

NCERT anchor: NCERT Joyful Mathematics Class 1 — Chapter 5: How Many Ate? and Chapter 9: How Many Times? (counting back)

What you'll learn

Subtract numbers up to 20, where the starting number can be a teen number.
Use the counting back strategy on a number line.
Solve subtraction facts confidently without always needing pictures.

Key concepts

1. Counting back strategy

For 15 − 4: start at 15, count back 4 steps → 14, 13, 12, 11. Answer: 11.

Counting back is faster than crossing out 15 dots one by one.

2. Using a number line

Draw a number line from 1 to 20. Start at the bigger number and hop backwards the number of times shown by the smaller number.

3. Subtracting teen numbers

18 − 5: think of 18 as 1 ten and 8 ones. Take 5 away from the 8 ones: 3 ones left, plus 1 ten = 13.

4. Checking your answer

Add the answer back to the number you subtracted. If 15 − 4 = 11, check: 11 + 4 = 15 ✓

Worked example

Counting back cricket balls

Step 1 — There are 16 cricket balls in the bag.
Step 2 — The coach takes out 7 balls for practice.
Step 3 — Start at 16, count back 7 steps: 15, 14, 13, 12, 11, 10, 9.
Step 4 — Write 16 − 7 = 9.
Step 5 — Check: 9 + 7 = 16 ✓
Answer: 9 cricket balls are left in the bag.

Common mistakes

Mistake	Why	Fix
Counting back one step too many or too few	Losing track of the hops	Use fingers or tally marks to count each backward step
Starting from the smaller number	Forgetting which number to start at	Always start counting back from the bigger number
Forgetting to check	Skipping verification	Add the answer to the number subtracted — it should equal the start

Quick check

What is 14 − 6?
Start at 19. Count back 5. Where do you stop?
There are 20 flags on a string. 8 flags are removed. How many flags are left?
Stretch: Priya had 17 marbles. She lost 5 marbles on Monday and 3 more on Tuesday. How many marbles does she have now?

Revision tip: Practise counting backwards from 20 to 1 every day — it makes counting back for subtraction much faster.

Open the Practice tab for graded questions on Subtract Within 20.

Subtract Within 20