Year 4

We can add and subtract mentally pairs of two-digit numbers and find a difference by counting on.
We can make jottings to support mental calculations.

Year 4 Unit 10

What we are learning:

• Addition can be done in any order. The order chosen should reflect the numbers in the addition. Always look for the simplest order to add the numbers together.
• Subtraction cannot be done in any order.
• When adding (mentally) a pair of 2-digit numbers the strategies that can be used include:
  – Keep largest number whole, partition second number into tens and units. First add tens, then add units. i.e. to calculate 47+58, keep 58 whole then add 40 (=98) then add 7 (=105).
  – Partition both numbers into tens and units. Add tens together, add units together then re-combine tens and units. i.e.to calculate 47 + 58, add tens 40+50 (=90) add units 7+8 (=15) then recombine tens and units 90+15 (=105).
  – If one number is very ‘close’ to a multiple of 10, adjust the units so that it becomes a multiple of 10, then add second number. i.e. to calculate 47+58, 58 is close to 60. Take 2 units from 47 (leaving 45) and add them first to 58 (making 60). The new calculation is now 45+60.
• When subtracting (mentally) a pair of 2-digit numbers the strategies that can be used are more limited than for addition:
  – Keep largest number whole, partition second number into tens and units. First subtract tens, then subtract units. i.e. to calculate 91 – 35, keep 91 whole then subtract 30 (=61) then subtract 5 this may need to be done in two stages as -1 then -4 (=56).
  – An alternative method for subtraction is to find the difference between the two numbers by counting up from the lowest value to the highest value. i.e. to calculate 91-35, start on 35 add 5 (to get to 40) add 50 (to get to 90) add 1 (to get to 91). In total 56 has been added, so 91-35=56.

A jotting might be no more than writing down the numbers in the calculation and the ‘sub-totals’ calculated along the way. A jotting may also be a number line. A number line is a simple diagram (quick to draw – no need for scaling
accuracy) used to represent and support a calculation. It consists of a straight line (normally drawn horizontally). Key numbers should be added onto it.

ACTIVITY 1: WHICH METHOD?

ACTIVITY SHEETS

Downloads:

Activity sheet: Which Method PDF

ACTIVITY 2: FINDING WAYS TO ADD MENTALLY

Activities you can do at home:

Look together at the sheet “Which method?” Decide upon the best strategy to use to solve each calculation.

Use an Empty Number Line to support the calculations on “Which method?” See the example below:

91-47 = 44

Good questions to ask and discuss:

Why is it helpful to be able to add and subtract numbers in our head?
What types of jottings do you find most helpful?

If your child:

Is not sure what they ‘need’ to write down
Reassure them that they can jot down anything they need to – there is not one ‘correct’ method at this stage, we are encouraging children to use approaches that suit them and that they understand. Focus on asking your child to explain what they are writing down and how it helps them.