Page 24 - Whole Numbers (Addition and Subtraction)

When we have to calculate 843 − 385 and we break down 843
and 385, we find that the parts are not convenient for calculating
843 − 385:
843 = 800 + 40 + 3
385 = 300 + 80 + 5

Fortunately we can make a plan:

We can replace 800 + 40 + 3 with 700 + 130 + 13.

843 = 700 + 130 + 13
385 = 300 + 80 + 5
It is now easy to work with the parts, that is to subtract the
hundreds parts, the tens parts and the units parts:
700 − 300 = 400 130 − 80 = 50 13 − 5 = 8

Now you can build up the answer:

843 − 385 = 400 + 50 + 8, which is 458.

9. Use the break down and build up method to calculate these.
(a) 934 − 676 (b) 845 − 469
(c) 348 − 165 (d) 952 − 387

10. Do the calculations in question 9 by using the filling-up
method. Then compare the answers that you got with the two
methods to check whether you worked correctly.

11. Use any method that you prefer to do each calculation below.
Use addition to check your answers.
(a) 876 − 345 (b) 766 − 232
(c) 734 − 476 (d) 624 − 387

12. Janice and Zain both calculate 760 − 340 + 260.
Janice first calculates 760 − 340; then she adds 260.
Zain first calculates 340 + 260; then he subtracts from 760.

(a) Do you expect them to get the same answer?
(b) Investigate whether your expectation is right.

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