1. 口算。(8分)
$8.6 + 6.4 =$ $10 - 7.25 =$ $3.6 - 1.7 =$
$36 ÷ 9 × 8 =$ $120 × 8 =$ $720 ÷ 90 =$
$80 × 50 =$ $25 × 4 ÷ 50 =$ $0.6 + 6.04 =$
$9.5 - 8.71 =$ $0.686 × 100 =$ $420 ÷ 7 ÷ 6 =$
$12.3 ÷ 100 =$ $7 ÷ 1000 =$ $0.5 × 10 =$
$125 × 9 × 8 =$

1. $8.6 + 6.4 = 15$
2. $10 - 7.25 = 2.75$
3. $3.6 - 1.7 = 1.9$
4. $36 ÷ 9 × 8 = 32$
5. $120 × 8 = 960$
6. $720 ÷ 90 = 8$
7. $80 × 50 = 4000$
8. $25 × 4 ÷ 50 = 2$
9. $0.6 + 6.04 = 6.64$
10. $9.5 - 8.71 = 0.79$
11. $0.686 × 100 = 68.6$
12. $420 ÷ 7 ÷ 6 = 10$
13. $12.3 ÷ 100 = 0.123$
14. $7 ÷ 1000 = 0.007$
15. $0.5 × 10 = 5$
16. $125 × 9 × 8 = 9000$

2. 用竖式计算并验算。(4分)
$10.5 + 6.88$ $35.2 - 16.75$

① $10.5 + 6.88 = 17.38$
竖式：
   1 0. 5 0
 +   6. 8 8
 ——————
   1 7. 3 8
验算：
$17.38 - 6.88 = 10.5$
  1 7. 3 8
6. 8 8
 ——————
  1 0. 5 0

② $35.2 - 16.75 = 18.45$
竖式：
 3 5. 2 0
1 6. 7 5
 ——————
 1 8. 4 5
验算：
$18.45 + 16.75 = 35.2$
 1 8. 4 5
+ 1 6. 7 5
 ——————
 3 5. 2 0

3. 怎样简便就怎样计算。(18分)
$73 + 45 + 127$ $19.35 - 18.6 + 10.65$
$3.85 - 1.23 - 0.77$ $5.2 × 75 + 25 × 5.2$
$63 × 125 × 8$ $43 × 101$ $99 × 65$
$21 × (200 - 180) × 5$ $9000 ÷ 125 ÷ 8$

3. 怎样简便就怎样计算。
1. $73 + 45 + 127$
$= (73 + 127) + 45$
$= 200 + 45$
$= 245$
2. $19.35 - 18.6 + 10.65$
$= (19.35 + 10.65) - 18.6$
$= 30 - 18.6$
$= 11.4$
3. $3.85 - 1.23 - 0.77$
$= 3.85 - (1.23 + 0.77)$
$= 3.85 - 2$
$= 1.85$
4. $5.2 × 75 + 25 × 5.2$
$= 5.2 × (75 + 25)$
$= 5.2 × 100$
$= 520$
5. $63 × 125 × 8$
$= 63 × (125 × 8)$
$= 63 × 1000$
$= 63000$
6. $43 × 101$
$= 43 × (100 + 1)$
$= 43 × 100 + 43 × 1$
$= 4300 + 43$
$= 4343$
7. $99 × 65$
$= (100 - 1) × 65$
$= 100 × 65 - 1 × 65$
$= 6500 - 65$
$= 6435$
8. $21 × (200 - 180) × 5$
$= 21 × 20 × 5$
$= 21 × (20 × 5)$
$= 21 × 100$
$= 2100$
9. $9000 ÷ 125 ÷ 8$
$= 9000 ÷ (125 × 8)$
$= 9000 ÷ 1000$
$= 9$