2.78 + 0.22 = 3

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The ability to work out addition and subtraction problems is very important in all walks of life. It's used when stock-taking for example. A bakery has 100 iced buns and sells 60 by lunchtime. In the afternoon a customer requests 38 iced buns for a party. The baker needs to know whether she has enough buns left so she must be able to work out the answer. Or you might want to work out the year in which your grandmother was born. If she is 60 years old then you can take 60 from the current year to get the answer. Both of these are examples of addition and subtraction problems.

1.
Which of these number sentences is incorrect?
2.56 + 3.41 = 5.97
5.97 - 3.41 = 2.56
5.97 - 2.56 = 3.41
3.41 - 2.56 = 5.97
3.41 - 2.56 = 0.85
2.
Which of these pairs would not make a whole number?
2.78 + 0.22
3.54 + 2.46
5.01 + 1.9
4.97 + 2.03
5.1 + 1.9 = 7 but 5.01 + 1.9 = 6.91 which is not a whole number
3.
Which strategy would be best to subtract 2.9?
Subtract 3 subtract 0.1
e.g. 3.4 - 2.9:
3.4 - 3 = 0.4
0.4 + 0.1 = 0.5
so 3.4 - 2.9 = 0.5
4.
What is the best method of adding 0.9?
Subtract 1 subtract 0.1
e.g. 1.3 + 0.9:
1.3 + 1 = 2.3
2.3 - 0.1 = 2.2
so 1.3 + 0.9 = 2.2
5.
Increase 450 by 230.
220
280
580
680
Increase means get larger so we add the numbers
6.
What is the best method of adding 34 + 48 + 56?
Find the pair that total a multiple of 100 - add these first
Find the pair that total a multiple of 10 - add these first
Add 10s first then the units
34 + 56 = 90 which is a multiple of 10 now add 48 so 90 + 48 = 138
7.
Which strategy would be best to add 69 + 61 + 65 + 67?
(60 x 4) + (1 + 5 + 7 + 9)
(70 x 4) - (1 + 5 + 7 + 9)
(61 + 65) + (67 + 69)
(60 x 4) - (1 + 5 + 7 + 9)
60 x 4 = 240 and 1 + 5 + 7 + 9 = 22
240 + 22 = 262
8.
How many altogether are 258 and 162?
320
400
420
450
One way to work this out is to add the units: 2 + 8 = 10
then the tens: 50 + 60 = 110
then the hundreds: 200 + 100 = 300
then add them all together: 300 + 110 + 10 = 420
9.
What is the difference between 6.3 and 2.4?
2.9
3.9
4
4.1
Difference means take the smaller number away from the larger
10.
Decrease 2.5 by 1.8.
0.6
0.7
1.7
4.3
Decrease means get smaller so we subtract the numbers
Author:  Amanda Swift