Grade 2 Mathematics Subtraction – Subtraction Of 1 From Numbers With Two Digits Notes
Mathematics — Subtraction
Subtopic: Subtraction of 1 from Two‑Digit Numbers (age 7, Kenya)
Subtraction of 1 means “one less than” a number. We look at two‑digit numbers (10 to 99). There are two simple rules to use.
Rules
- If the ones digit is not 0 (for example 34), just take away 1 from the ones. 34 − 1 = 33.
- If the ones digit is 0 (for example 20), take one ten away and make the ones 9. 20 − 1 = 19.
Visual idea — Tens and Ones
We show a ten as a long block and ones as small circles.
Example 1: 34 − 1
10
10
10
Take away one small circle (one):
10
10
10
So 34 − 1 = 33.
Example 2: 20 − 1 (ones = 0)
Start: two tens and zero ones
10
10
(no ones)
Rule: since ones = 0, take one ten away. The remaining ten gives 9 ones.
10
So 20 − 1 = 19.
Number line method (quick)
To do −1 on a number line, move one step left.
Example: 42 − 1
40
41
42
43
44
45
42
41
Move one step left: 42 − 1 = 41
Steps you can follow every time
- Look at the ones digit (rightmost digit).
- If it is not 0, subtract 1 from the ones and keep the tens the same.
- If it is 0, reduce the tens by 1 and write 9 in the ones place.
Practice — write answers
- 25 − 1 = ______
- 41 − 1 = ______
- 70 − 1 = ______
- 88 − 1 = ______
- 60 − 1 = ______
- 13 − 1 = ______
- 30 − 1 = ______
- 99 − 1 = ______
Answers
1) 24 2) 40 3) 69 4) 87 5) 59 6) 12 7) 29 8) 98
Simple Kenyan word problems
- Juma had 31 mangoes. He gave 1 mango to a friend. How many mangoes remain? (31 − 1 = ?)
- A shop had 50 coins of 1 shilling each. One coin was lost. How many coins are left? (50 − 1 = ?)
Tip for parents and teachers: let the child use real objects (stones, beads, small wrappers) to show tens and ones. Practise both kinds of numbers: those whose ones are not 0 and those ending with 0 (like 20, 30, 40).