1.
a. 4 x 14 = 56
b. 50 x 200 = 10 000
(But 50 x 190 = 9500 is wrong)
c. 34 x 400 = 13 600
d. 11 x 80 = 880
e. 13 x 4 = 52
f. 100 / 20 = 5
g. 40 / 2 =20
h. 70 / 7 =10
i. 12 / 6 = 2
(But 10 / 6 = 1.67 is wrong!)
2.
a.
4  20
80

 20  16  4 (Note: Actual value is 4.011)
4
4
b.
82  7 2  64  49  113  121  11 (Note: Actual value is 10.918)
c.
64  20
 64  4  8  2  16. THIS IS CORRECT!
5
BUT
60  20
1200

 240  16  15  4 15  15.49 IS WRONG! We can't have decimal.
5
5
(Note: Actual value is 15.587)
d.
10  20
200

 50  49  7
4
4
(Note: Actual value is 7.70)
3.
a. 15 x 400 = 6000 which means that James' answer is approximately one tenth of the
correct value.
b. 14.62 x 401 = 5862.62. James has put the decimal point in the wrong place.
4. a. This calculation is incorrect! Since 15 x 6000 = 90 000 (exact is 93 024 not 930 240)
b. This calculation is correct! The answer 5 or 6 is close to 5.26.
60 / 12 = 5 or 60 / 10 = 6 (exact is 5.26 which is given)
But 70 / 10 = 7 is wrong. This is higher than 5.26.
c. This is correct! Since 200 x 4600 = 920 000.
or 190 x 4600 = 874 000. (exact is 880 704 which is given)
d. This calculation is incorrect! Since 350 / 2 = 175 which is higher than 14.7. (exact is 147)
5. a. 4 x 800 = 3200. So the correct answer must be 2988.16.
b. 3000 / 10 = 300. So the correct answer must be 301
c.
7 2  82

4
d.
80  70 150

 75. So the correct answer must be 71.73
2
2
49  64
113

 28.25  25  5 . So the correct answer must be 5.4187
4
4
6. Number of seconds in December = 31×24×60×60 ≈ 2 400 000
The correct number of seconds in December = 2 678 400
7. π × 20.5 ≈ 3 × 20 = 60 cm
8. Number of words in book = 326 × 268.4 ≈ 300 × 300 = 90 000 words
9. Total spent in canteen = 1264 × £1.42 ≈ 1200 × £1.50 = £1800
10. Area = π × 6.27² ≈ 3 × 6² = 3 × 36 = 108 cm²
11. (28 x 4) - 22 ≈ (30 x 4) - 20 ≈ 120 - 20 ≈ 100