Add and subtract 1 to and from a 2-digit number crossing the tens boundary | Maths

Starter quiz

Which of the following two numbers have a difference of 1?

57 and 59

34 and 36

89 and 88 ✓
Sam gathers Base 10 blocks to show one more than 47 Which image shows his Base 10 blocks?
Which number completes the stem sentences? 58 is equal to ______ plus 1

'57' ✓
Choose which equation represents this part-part-whole model, then solve it.

69 - 1 = ___ ✓

69 + 1 = ___
Alex represents this number. Which place value chart represents the number that is one less?
Solve each equation by matching correctly.

___ = 34 + 1

⇔

35 ✓

34 = ___ + 1

⇔

33 ✓

___ - 1 = 34

⇔

35 ✓

Exit quiz

Which decade is missing on the number line?

the fifties ✓

the sixties

the forties
Alex adds 1 one to these Base 10 blocks. Which picture shows the number he now has?
Alex taps one less than the number shown. What number does he tap?

'59' ✓
Sam thinks of two numbers with a difference of 1 One of her numbers is 30 What could the other number be?

'31' ✓
Which numbers are missing?

61 and 62

62 and 63

59 and 60 ✓
Which equation represents the relationship between the numbers on the number line.

59 + 60 = 1

59 + 1 = 60 ✓

60 - 1 = 59 ✓

60 - 59 = 1

Worksheet

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Lesson Details

Key learning points

Finding one more is the same as adding one; finding one less is the same as subtracting one.
Numbers that are one more or one less than each other have a difference of one.
When one is added to a number with a ones digit of 9, the next multiple of ten will be reached.
When one is subtracted from a multiple of ten, the number reached will have a ones digit of 9
When one ten is changed for ten ones, or ten ones for one ten, it is called regrouping.

Common misconception

Children may have difficulty finding one less than a multiple of ten saying, for example, one less than 60 is 69.

Refer to the order of the decades counting forwards and backwards and encourage visualisation of the decades on number lines and one hundred squares to support this. Help children to generalise about the number before a decade number ending in 9

Keywords

One more - One more is when there is a greater amount by one or another item in the set.
One less - One less is when there is a smaller value by one.
Difference - The result of subtracting one number from another, How much one number differs from another.
Decade - A group of ten numbers below one hundred, each with the same tens digit.
Regroup - To exchange ten ones for one ten or one ten for ten ones in order to calculate more easily.