Page 17 - Whole Numbers (Addition and Subtraction)

A different method to add multiples of 10 is to break numbers
down into hundreds parts and tens parts, for example:
260 + 90 = 200 + 60 + 90
= 200 + 150
= 200 + 100 + 50
= 300 + 50
= 350
You need not write down all the steps when you work like this.

3. Calculate each of the following in two ways.
Do it by breaking numbers down into hundreds and tens.
Also do it by filling up to a multiple of hundred.
Show your thinking clearly in writing in each case.

(a) 270 + 70 (b) 660 + 80 (c) 350 + 280
4. Calculate each of the following in the way that you prefer.
You may prefer different methods for different calculations.
(a) 780 + 30 (b) 580 + 230 it sometimes makes
(c) 470 + 60 (d) 60 + 230 addition easier if you
(e) 630 + 90 (f) 420 + 340 switch the numbers
(g) 480 + 340 (h) 70 + 380 around.
(i) 730 + 100 (j) 730 + 160

Addition can be used to check
whether subtraction was done
correctly.

You can calculate 940 − 380 by adding on to 380 until you reach
940, as shown here:
380 + 20 → 400 + 540 = 940
So 940 − 380 = 20 + 540, which is 560.
You can also subtract by rounding off and compensating.
For example, you can calculate 830 − 270 by first subtracting 300:
830 − 300 = 530 but you have now subtracted 30 too much.
So you have to compensate by putting the 30 back:
530 + 30 = 560, so 830 − 270 = 560.

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