1. © T Madas

2. Estimate:
9 x 22 ≈ 10 x 20 =
200
[198]
11 x 19 ≈ 10 x 20 =
200
[209]
21 x 31 ≈ 20 x 30 =
600
[651]
32 x 49 ≈ 30 x 50 =
1500
[1568]
71 x 306 ≈ 70 x
300 =
21000
[21726]
98 x 123 ≈
100 x
120 =
12000
[12054]
4.1 x 5.7 ≈ 4 x 6 = 24
[23.37]
2.1 x 7.8 ≈ 2 x 8 = 16
[16.38]
1.11 x 18.7 ≈ 1 x 19 = 19
[20.757]
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3. Estimate: 212 ÷ 22 ≈ 200 ÷ 20 = 10 [ 9.64 2 d.p.] 715 ÷ 96 ≈ 700 ÷ 100
3955 ÷ 79 ≈
4000 ÷ 80 = 50
[ d.p.]
8934 ÷ 31 ≈
9000 ÷ 30 =
300
[ d.p.]
5.2 ÷ 1.12 ≈ 5 ÷ 1 = 5
[ d.p.]
9.21 ÷ 8.89 ≈ 9 ÷ 9 = 1
[ d.p.]
© T Madas

4. Estimate: 29 x 83 30 x 80
2400 ≈ = = 60
[ d.p.] 42 40 40 39 x
602 40 x
600
24000 ≈ = =
120
[ d.p.]
201
200
200 21 x
885 20 x
900
18000 ≈ = = 60
[ d.p.]
296
300
300 52 x
406 50 x
400
20000 ≈ = = 80
[ d.p.]
257
250
250
© T Madas

5. Estimate:
153 x
10.12
153 x 10 ≈ =
153
x 2 =
306
[ d.p.]
4.96 5
18.8 x
20.44 19 x 20 ≈ = 19
x 4 = 76
[ d.p.]
5.12 5
17.7 x
122 18 x
120 ≈ = 3
x 120 =
360
[ d.p.]
6.11 6
1.53 x
12.91
1.5 x 13 ≈ = 3
x 13 = 39
[ d.p.]
0.49
0.5
© T Madas

6. Estimate: 57 x
8.99 60 x 9
540 54 27 ≈ = = =
= 2.7
19.75 x 10.22 20 x 10
200 20 10
[ d.p.] 44 x
153 45 x
150 45 x
150 ≈ = = 3
x 150
= 450
147 ÷ 10.02
150 ÷ 10 15
[ d.p.] 61 +
59.9 x
8.99 60 + 60 x 9 60 +
540
600 ≈ = =
= 2 –
202 –
102
400 –
100
300
[ d.p.]
© T Madas

7. © T Madas

8. Estimate:
9 x 22 ≈ 10 x 20 =
200
[198]
11 x 19 ≈ 10 x 20 =
200
[209]
21 x 31 ≈ 20 x 30 =
600
[651]
32 x 49 ≈ 30 x 50 =
1500
[1568]
71 x 306 ≈ 70 x
300 =
21000
[21726]
98 x 123 ≈
100 x
120 =
12000
[12054]
4.1 x 5.7 ≈ 4 x 6 = 24
[23.37]
2.1 x 7.8 ≈ 2 x 8 = 16
[16.38]
1.11 x 18.7 ≈ 1 x 19 = 19
[20.757]
© T Madas

9. © T Madas

10. Estimate: 212 ÷ 22 ≈ 200 ÷ 20 = 10 [ 9.64 2 d.p.] 715 ÷ 96 ≈ 700 ÷ 100
3955 ÷ 79 ≈
4000 ÷ 80 = 50
[ d.p.]
8934 ÷ 31 ≈
9000 ÷ 30 =
300
[ d.p.]
5.2 ÷ 1.12 ≈ 5 ÷ 1 = 5
[ d.p.]
9.21 ÷ 8.89 ≈ 9 ÷ 9 = 1
[ d.p.]
© T Madas

11. © T Madas

12. Estimate: 29 x 83 30 x 80
2400 ≈ = = 60
[ d.p.] 42 40 40 39 x
602 40 x
600
24000 ≈ = =
120
[ d.p.]
201
200
200 21 x
885 20 x
900
18000 ≈ = = 60
[ d.p.]
296
300
300 52 x
406 50 x
400
20000 ≈ = = 80
[ d.p.]
257
250
250
© T Madas

13. © T Madas

14. Estimate:
153 x
10.12
153 x 10 ≈ =
153
x 2 =
306
[ d.p.]
4.96 5
18.8 x
20.44 19 x 20 ≈ = 19
x 4 = 76
[ d.p.]
5.12 5
17.7 x
122 18 x
120 ≈ = 3
x 120 =
360
[ d.p.]
6.11 6
1.53 x
12.91
1.5 x 13 ≈ = 3
x 13 = 39
[ d.p.]
0.49
0.5
© T Madas

15. © T Madas

16. Estimate: 57 x
8.99 60 x 9
540 54 27 ≈ = = =
= 2.7
19.75 x 10.22 20 x 10
200 20 10
[ d.p.] 44 x
153 45 x
150 45 x
150 ≈ = = 3
x 150
= 450
147 ÷ 10.02
150 ÷ 10 15
[ d.p.] 61 +
59.9 x
8.99 60 + 60 x 9 60 +
540
600 ≈ = =
= 2 –
202 –
102
400 –
100
300
[ d.p.]
© T Madas

17. © T Madas

18. Estimate:
15.3 x
10.12
15.3 x 10 ≈ =
153
x 2 =
306
[ d.p.]
0.496
0.5
19.2 x
5.34 19 x 5 ≈ = 95
x 10 =
950
[ d.p.]
0.11
0.1
7.9 x
15.2 8 x 15 ≈ =
120
x 4 =
480
[ d.p.]
0.24
0.25
2.53 x
11.91
2.5 x 12 ≈ = 30
x 5 =
150
[ d.p.]
0.19
0.2
© T Madas

19. © T Madas

20. Fifi estimates the value of to be 6.
Write down the 3 numbers Fifi used in this estimate.
Without performing any calculations explain why the value must be more than 6.
20.7 x
61.5
187
20.7 x
61.5 20 x 60 ≈
187
200
The value of 6 obtained, is an underestimate because:
both numbers on the numerator have been rounded down
the number in the denominator has been rounded up
© T Madas

21. © T Madas