【 ⼩數乘法 】
8 x 9 = 72
8 x 0.9 = 7.2
8 x 0.09 = 0.72
0.8 x 9 = 7.2
0.8 x 0.9 = 0.72
# 先按平常沒有⼩數點計算，再按原本乘式中⼩數點位調整答案⼤⼩
3.2 x 3 => 32 x 3 = 96 , 所以 3.2 x 3 ＝9.6
4.11 x 2 => 411 x 2 = 822 , 所以 4.11 x 2 ＝8.22
0.22 x 0.3 => 22 x 3 = 66 , 所以 0.22 x 0.3 ＝0.066
【 互相抵消 】
10 x 0.1 = 1
100 x 0.01 = 1
# ⼀個 0 抵消⼀個⼩數點位
1000 x 0.001 = 1
10 x 3 = 30
10 x 0.3 = 3
10 x 0.9 = 9
10 x 0.04 = 0.4
100 x 3 = 300
100 x 0.3 = 30
100 x 0.03 = 3
100 x 0.9 = 90
100 x 0.09 = 9
40 x 0.6 => 4 x 10 x 0.6 => 4 x 6 , 所以 40 x 0.6 = 24
50 x 0.7 => 5 x 10 x 0.7 => 5 x 7 , 所以 50 x 0.7 = 35
200 x 0.3 => 2 x 100 x 0.3 => 5 x 7 , 所以 50 x 0.7 = 35
600 x 0.13 = 6 x 13 = 78
300 x 0.09 = 3 x 9 = 27
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800 ÷ 200
1500 ÷ 300
6000 ÷ 400
＝8÷2
＝ 15 ÷ 3
＝ 60 ÷ 4
＝4
＝5
＝ 15
3 ÷ 10 ＝ 0.3
0.3 ÷ 10 ＝ 0.03
0.3 ÷ 100 ＝ 0.003