Page 19 - Whole Numbers (Addition and Subtraction)

7. Break down, work with parts and build up the answer to
calculate the following.
(a) 430 − 160 (b) 760 − 280

(c) 630 − 290 (d) 420 − 340
8. Check whether you got the same answers for the above
calculations as when you did them in question 6.

9. Use any method that you prefer to calculate the following.
Check all your answers by doing addition.
(a) 810 − 220 (b) 710 − 120

(c) 630 − 330 (d) 630 − 460
(e) 640 − 370 (f) 820 − 280

10. Lerato calculated 840 − 550 like this:
840 − 540 = 300 so 840 − 550 is 10 less than 300,
and that is 290.

Do you think Lerato was clever to work like this?
Write a short paragraph about it.

3.7 Round off and estimate

It is often useful to quickly make approximate answers for
calculations. Imagine you buy items for R34 and R58 and do not
have time to accurately calculate R34 + R58.

It can help you to know that you need to pay about
R30 + R60, which is R90.

34 can be rounded off to the 30 40
nearest multiple of ten,
which is 30.
50 60
58 can be rounded off to 60.
55 is equally far from 50 and 60. 50 55 60
People all over the world have agreed to round off “upwards” in a
case like this, so 55 is normally rounded off to 60.

18