1. 口算。
$600×7= $ $21×3= $ $30×8= $ $71×9= $
$156-56= $ $74+36= $ $125+50= $ $378-108= $
$0×8= $ $210×2= $ $400×5= $ $90×8= $
$\frac{6}{7}-\frac{3}{7}= $ $1-\frac{3}{8}= $ $\frac{5}{6}-\frac{5}{6}= $ $\frac{3}{4}+\frac{1}{4}= $

$600×7=4200$ $21×3=63$ $30×8=240$ $71×9=639$
$156 - 56 = 100$ $74 + 36 = 110$ $125 + 50 = 175$ $378 - 108 = 270$
$0×8 = 0$ $210×2 = 420$ $400×5 = 2000$ $90×8 = 720$
$\frac{6}{7}-\frac{3}{7}=\frac{3}{7}$ $1-\frac{3}{8}=\frac{5}{8}$ $\frac{5}{6}-\frac{5}{6}=0$ $\frac{3}{4}+\frac{1}{4}=1$

2. 用竖式计算。
$298×5$ $270×3$ $854-409$ $107×7$

1490　810　445　749

3. 脱式计算。
$321×3-158$ $572-(156+200)$
$76+24×5$ $104×(56-49)$

$321×3 - 158 = 963 - 158 = 805$
$572 - (156 + 200) = 572 - 356 = 216$
$76 + 24×5 = 76 + 120 = 196$
$104×(56 - 49) = 104×7 = 728$