Exercise 2.7 (multiplication facts by adding partial products mentally)

Exercise 2.7

Simplify the following multiplication facts by adding partial products.

1. 9 x 42

2. 3 x 78

3. 36 x 12

4. 18 x 19

5. 68 x 31

6. 42 x 21

1. 9 x 42

42 = 40 + 2

9 x 40 = 360

9 x 2 = 18

360 + 18 = 378

Answer: 378

2. 3 x 78

78 = 70+8

3 x 70 = 210

3x 8 = 24

210 + 24= 234

Answer: 234

3. 36 x12

36 = 30 + 6

12 = 10+2

30 x10 = 300

30 x 2 = 60

6 x 10 =60

6 x 2 = 12

300 + 60 +60 +12 = 432

Answer: 432

4. 18 x 19

18 = 10+8

19 = 10+9

10x10 =100

10 x 9 = 90

8 x10 = 80

8 x 9 = 72

100 +90+80+72=342

Answer: 342

5. 68 x 31

68 = 60+8

31 = 30+ 1

60 x30 = 1800

60 x 1 = 60

8 x 30 = 240

8 x 1 =8

1800+60+240+8=2108

Answer: 2108

6. 42 x 21

42 = 40 + 2

21 = 20+1

40 x20 = 800

40 x 1 = 40

2 x20 = 40

2 x 1 = 2

800+40+40+2=882

Answer: 882