扬帆起航
$3.5 - 1.5 + 0.9 =$ $7 - 2.6 + 5.6 =$ $32.7 - 12.7 + 0.8 =$
$9.7 + 1.3 + 0.8 =$ $8.1 + 0.9 - 2.5 =$ $2.99 + 0.01 + 5.4 =$
$30.3 + 6.7 - 8 =$ $5.7 - 1.7 + 0.8 =$ $9.8 - 2.8 + 0.5 =$
$6.3 - 5.1 - 0.3 =$ $20 - 3.7 + 2.8 =$ $4.25 + 5.75 - 1.9 =$

1.
$3.5 - 1.5 + 0.9$
$=2 + 0.9$
$=2.9$
2.
$7 - 2.6 + 5.6$
$=4.4 + 5.6$
$=10$
3.
$32.7 - 12.7 + 0.8$
$=20 + 0.8$
$=20.8$
4.
$9.7 + 1.3 + 0.8$
$=11+ 0.8$
$=11.8$
5.
$8.1 + 0.9 - 2.5$
$=9 - 2.5$
$=6.5$
6.
$2.99 + 0.01 + 5.4$
$=3 + 5.4$
$=8.4$
7.
$30.3 + 6.7 - 8$
$=37 - 8$
$=29$
8.
$5.7 - 1.7 + 0.8$
$=4 + 0.8$
$=4.8$
9.
$9.8 - 2.8 + 0.5$
$=7 + 0.5$
$=7.5$
10.
$6.3 - 5.1 - 0.3$
$=1.2 - 0.3$
$=0.9$
11.
$20 - 3.7 + 2.8$
$=16.3 + 2.8$
$=19.1$
12.
$4.25 + 5.75 - 1.9$
$=10 - 1.9$
$=8.1$

乘风破浪
$0.9 + 0.5 - 0.5 =$ $9 - (7 + 1.99) =$
$1.7 + (10.2 - 0.5) =$ $8.5 + 0.5 - 2.7 =$
$8.2 + 1.4 - 2.5 =$ $1.39 - 0.61 + 4.6 =$
$6.1 - (7.1 - 3.55) =$ $9.8 + (5 - 4.39) =$
$16.7 + (5.9 - 5.6) =$ $9.2 + 1.8 - 3.6 =$
$8.5 + 1.8 - 0.6 =$ $19.5 + (2.7 + 0.3) =$
$20 - 3.5 - 2.5 =$ $100 - 3.8 + 2.2 =$
$0.65 - (0.14 + 0.08) =$ $7.7 - (2.4 + 2.6) =$
$19 - (5.8 + 6.2) =$ $17.1 + (0.15 + 0.85) =$

1. $0.9 + 0.5 - 0.5$
$= 1.4 - 0.5$
$= 0.9$
2. $9 - (7 + 1.99)$
$= 9 - 8.99$
$= 0.01$
3. $1.7 + (10.2 - 0.5)$
$= 1.7 + 9.7$
$= 11.4$
4. $8.5 + 0.5 - 2.7$
$= 9 - 2.7$
$= 6.3$
5. $8.2 + 1.4 - 2.5$
$= 9.6 - 2.5$
$= 7.1$
6. $1.39 - 0.61 + 4.6$
$= 0.78 + 4.6$
$= 5.38$
7. $6.1 - (7.1 - 3.55)$
$= 6.1 - 3.55$
$= 2.55$
8. $9.8 + (5 - 4.39)$
$= 9.8 + 0.61$
$= 10.41$
9. $16.7 + (5.9 - 5.6)$
$= 16.7 + 0.3$
$= 17$
10. $9.2 + 1.8 - 3.6$
$= 11 - 3.6$
$= 7.4$
11. $8.5 + 1.8 - 0.6$
$= 10.3 - 0.6$
$= 9.7$
12. $19.5 + (2.7 + 0.3)$
$= 19.5 + 3$
$= 22.5$
13. $20 - 3.5 - 2.5$
$= 16.5 - 2.5$
$= 14$
14. $100 - 3.8 + 2.2$
$= 96.2 + 2.2$
$= 98.4$
15. $0.65 - (0.14 + 0.08)$
$= 0.65 - 0.22$
$= 0.43$
16. $7.7 - (2.4 + 2.6)$
$= 7.7 - 5$
$= 2.7$
17. $19 - (5.8 + 6.2)$
$= 19 - 12$
$= 7$
18. $17.1 + (0.15 + 0.85)$
$= 17.1 + 1$
$= 18.1$

圆满返航
数一数，右图中有()个三角形，()个四边形。

秘籍点拨 先数出一层有几个三角形，再数有几层这样的三角形，就可以求出三角形的总个数了。数图形的个数时，要注意有序地数哦！

12；6