一、口算。
$0.9 - 0.52 = $ $2.7×0.3 = $ $0.19 - 0.13 = $
$5÷40 = $ $0.16÷0.4 = $ $4.3 + 1.7 = $
$1.98×0.2 = $ $0.41×5 = $ $2.4×0.5 = $
$7.2÷9 = $ $9÷0.125 = $ $10 - 0.4 = $
$0.26 + 0.4 = $ $0.4×0.75 = $ $4.6 + 4 = $
$0.64÷0.8 = $ $0.081×20 = $ $0.9×0.3 = $
$\frac{1}{3} - \frac{1}{8} = $ $\frac{7}{20} + \frac{3}{5} = $ $\frac{9}{14} - \frac{2}{21} = $
$\frac{1}{4} + \frac{2}{9} = $ $\frac{7}{10} - \frac{2}{3} = $ $\frac{2}{5} + \frac{4}{9} = $

$0.9 - 0.52 = 0.38$；$2.7×0.3 = 0.81$；$0.19 - 0.13 = 0.06$；
$5÷40 = 0.125$；$0.16÷0.4 = 0.4$；$4.3 + 1.7 = 6$；
$1.98×0.2 = 0.396$；$0.41×5 = 2.05$；$2.4×0.5 = 1.2$；
$7.2÷9 = 0.8$；$9÷0.125 = 72$；$10 - 0.4 = 9.6$；
$0.26 + 0.4 = 0.66$；$0.4×0.75 = 0.3$；$4.6 + 4 = 8.6$；
$0.64÷0.8 = 0.8$；$0.081×20 = 1.62$；$0.9×0.3 = 0.27$；
$\frac{1}{3} - \frac{1}{8} = \frac{5}{24}$；$\frac{7}{20} + \frac{3}{5} = \frac{19}{20}$；$\frac{9}{14} - \frac{2}{21} = \frac{23}{42}$；
$\frac{1}{4} + \frac{2}{9} = \frac{17}{36}$；$\frac{7}{10} - \frac{2}{3} = \frac{1}{30}$；$\frac{2}{5} + \frac{4}{9} = \frac{38}{45}$。

二、计算下面各题,怎样简便就怎样计算。
$0.38 + 9.6÷3.2$ $8.6×0.6 + 8.6×0.4$ $3.75÷[3×(1.34 - 0.84)]$
$3.2×1.25÷0.8$ $(72 - 72×0.9)÷1.2$ $7.05 - 2.24 - 1.76 + 5.95$

1. 计算$0.38 + 9.6÷3.2$
解：
$\begin{aligned}&0.38 + 9.6÷3.2\\=&0.38 + 3\\=&3.38\end{aligned}$
2. 计算$8.6×0.6 + 8.6×0.4$
解：
根据乘法分配律$a× c + b× c=(a + b)× c$，这里$a = 0.6$，$b = 0.4$，$c = 8.6$。
$\begin{aligned}&8.6×0.6 + 8.6×0.4\\=&8.6×(0.6 + 0.4)\\=&8.6×1\\=&8.6\end{aligned}$
3. 计算$3.75÷[3×(1.34 - 0.84)]$
解：
先算小括号里的：$1.34−0.84 = 0.5$，再算中括号里的：$3×0.5 = 1.5$，最后算除法：
$\begin{aligned}&3.75÷[3×(1.34 - 0.84)]\\=&3.75÷(3×0.5)\\=&3.75÷1.5\\=&2.5\end{aligned}$
4. 计算$3.2×1.25÷0.8$
解：
根据乘除法的运算顺序，从左到右依次计算，先算$3.2×1.25$，把$3.2$拆分为$0.4×8$，则$3.2×1.25=0.4×(8×1.25)=0.4×10 = 4$，再算$4÷0.8$。
$\begin{aligned}&3.2×1.25÷0.8\\=&(0.4×8)×1.25÷0.8\\=&0.4×(8×1.25)÷0.8\\=&0.4×10÷0.8\\=&4÷0.8\\=&5\end{aligned}$
5. 计算$(72 - 72×0.9)÷1.2$
解：
根据乘法分配律的逆运算$a× c−b× c=(a - b)× c$，这里$a = 1$，$b = 0.9$，$c = 72$，先算括号里的$72−72×0.9 = 72×(1 - 0.9)=72×0.1$，再算除法。
$\begin{aligned}&(72 - 72×0.9)÷1.2\\=&72×(1 - 0.9)÷1.2\\=&72×0.1÷1.2\\=&7.2÷1.2\\=&6\end{aligned}$
6. 计算$7.05 - 2.24 - 1.76 + 5.95$
解：
根据加法交换律$a + b=b + a$和减法的性质$a - b - c=a-(b + c)$，可得：
$\begin{aligned}&7.05 - 2.24 - 1.76 + 5.95\\=&(7.05 + 5.95)-(2.24 + 1.76)\\=&13 - 4\\=&9\end{aligned}$
综上，答案依次为$3.38$；$8.6$；$2.5$；$5$；$6$；$9$。

1. $0.38 + 9.6÷3.2$
$=0.38 + 3$
$=3.38$
2. $8.6×0.6 + 8.6×0.4$
$=8.6×(0.6 + 0.4)$
$=8.6×1$
$=8.6$
3. $3.75÷[3×(1.34 - 0.84)]$
$=3.75÷[3×0.5]$
$=3.75÷1.5$
$=2.5$
4. $3.2×1.25÷0.8$
$=3.2×(1.25÷0.8)$
$=3.2×1.5625$
$=5$
5. $(72 - 72×0.9)÷1.2$
$=72×(1 - 0.9)÷1.2$
$=72×0.1÷1.2$
$=7.2÷1.2$
$=6$
6. $7.05 - 2.24 - 1.76 + 5.95$
$=(7.05 + 5.95) - (2.24 + 1.76)$
$=13 - 4$
$=9$