1.

4 + 4 + 4 + 4 + 4 = 20
Possible strategy: knowledge of table facts
4 x 5 = 20

2.

6 x 0 x 17 x 83 = 0
Possible strategy: knowledge of properties of zero
Any number x 0 = 0

3.

20 x 1000 = 20 000
Possible strategy: multiplication of numbers with base 10

4.

20 x 7 = 140
Possible strategy: multiplication by stages
20 x 7
= 2 x 10 x 7
= 2 x 70
= 70 + 70
= 140

5.

20 x 14 = 280
Possible strategy: doubling a known fact
Since 20 x 7 = 140
20 x 14
= 20 x 7 x 2
= 140 x 2
= 140 + 140
= 280

6.

30 x 5 = 150
Possible strategy: multiplication by stages
30 x 5
= 3 x 10 x 5
= 3 x 50
= 150

7.

8 x 50 = 400
Possible strategy: halving and doubling
8 x 50
= 4 x 100
= 400

8.

Double 33 = 66
Possible strategy:
2 x 33
Since 2 x 3 = 6, then 2 x 33 = 66

9.

99 x 3 = 297
Possible strategy: multiplication by rounding
99 x 3
= (100 x 3) - (1 x 3)
= 300 - 3
= 297

10.

Double 195
Possible strategy: repeated addition
2 x 195
= 195 + 195
= 390

11.

17 x 9 = 9 x 17 = 153
By knowing the commutative law, students can easily solve this problem.

12.

13 x 14 = 182
Mental computation is often made easier when a know fact is used as a starting point.
Since 13 x 13 = 169
13 x 14
= 13 x 13 + 13
= 169 + 13
= 182

13.

20 x 20 = 400
Possible strategy: knowledge of base ten
Since 2 x 2 = 4, then 20 x 20 = 400

14.

50 x 5 = 250
Possible strategy: multiplication in stages
50 x 5
= 5 x 10 x 5
= 25 x 10
= 250
50 is a special number in mental computation and many student will know simple multiples of 50 by heart.

15.

How many days in 3 weeks?
Possible strategy: using table knowledge
3 x 7 = 21

How many days in 100 weeks?
Possible strategy: multiplication by base ten
7 x 100 = 700

How many days in 50 weeks?
Possible strategy: using a known fact and halving
Since 7 x 100 = 700, then 7 x 50 = 700 ÷ 2 = 350

Students may also solve this question using their knowledge of day and weeks in a year. If students know that there are 364 days and 52 weeks in a year, then they may simply subtract 14 from 364.