Class 1 — Mathematics

Subtraction: Taking Away Within 20

Class 1

✓By the end of this lesson students will be able to understand subtraction as 'taking away' or 'how many are left'.
✓By the end of this lesson students will be able to identify and use the minus (-) sign for subtraction.
✓By the end of this lesson students will be able to perform subtraction of numbers within 20 using concrete objects, drawings, or counting back.
✓By the end of this lesson students will be able to write simple subtraction sentences.

Key concepts

What is Subtraction?

Subtraction means 'taking away' a certain number of things from a group. It tells us 'how many are left' or 'the difference' between two numbers. When we subtract, the number of things usually becomes smaller.

The Minus Sign (-)

The minus sign (-) is the symbol we use for subtraction. When you see this sign between two numbers, it means you need to take away the second number from the first number.

Example: 5 - 2

The Equals Sign (=)

The equals sign (=) means 'is equal to'. It shows the answer to our subtraction problem. For example, 5 - 2 = 3 means '5 take away 2 is equal to 3'.

Example: 5 - 2 = 3

Subtraction Within 20

We can subtract numbers up to 20. We can use our fingers, draw pictures, or count backwards to find the answer. For example, to find 15 - 3, we can start at 15 and count back 3 numbers: 14, 13, 12. So, 15 - 3 = 12.

Key facts to remember

1Subtraction means taking away.
2The symbol for subtraction is the minus sign (-).
3When you subtract, the number usually becomes smaller.
4Subtracting zero from a number means the number stays the same (e.g., 7 - 0 = 7).
5Subtracting a number from itself always gives zero (e.g., 9 - 9 = 0).
6We can use fingers, drawings, or counting back to subtract numbers within 20.

Worked examples

Example 1

There are 8 mangoes in a basket. You eat 3 mangoes. How many mangoes are left?

IStep 1: Understand the problem. We start with 8 mangoes and take away 3 mangoes.

IIStep 2: Write the subtraction sentence: 8 - 3 = ?

IIIStep 3: You can draw 8 mangoes and then cross out 3 of them.

IVStep 4: Count the mangoes that are not crossed out. There are 5 mangoes left.

Answer

5 mangoes are left.

Drawing pictures helps us to visualise 'taking away'.

Example 2

Subtract 6 from 14.

IStep 1: Write the subtraction problem: 14 - 6 = ?

IIStep 2: Start with the number 14.

IIIStep 3: Count back 6 steps from 14. (You can use your fingers or a number line).

IVStep 4: Counting back: 14 -> 13 (1st step) -> 12 (2nd step) -> 11 (3rd step) -> 10 (4th step) -> 9 (5th step) -> 8 (6th step).

Answer

14 - 6 = 8

Counting back is a useful method for subtraction.

Example 3

Rohan had 17 pencils. He gave 5 pencils to his friend. How many pencils does Rohan have now?

IStep 1: Identify the total number of pencils Rohan had: 17.

IIStep 2: Identify the number of pencils he gave away: 5.

IIIStep 3: Form the subtraction problem: 17 - 5 = ?

IVStep 4: We need to take away 5 from 17. We can count back 5 numbers from 17.

VStep 5: 17 -> 16 -> 15 -> 14 -> 13 -> 12. (We counted back 5 times).

Answer

Rohan has 12 pencils now.

Always read the word problem carefully to understand what is being asked.

Common mistakes

✗Confusing subtraction with addition (e.g., adding instead of taking away).
✗Incorrectly counting back or forward when subtracting.
✗Not understanding the meaning of 'taking away' in word problems.
✗Making errors in counting the remaining objects or steps.

Exam tips

★Always read the question carefully to understand if you need to add or subtract.
★Use your fingers or draw simple pictures to help you solve subtraction problems.
★For 'taking away', you can cross out the items you are removing.
★Practice counting backwards from different numbers to become good at subtraction.

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