扬帆起航
$24×3=$ $2.4×3=$ $0.24×3=$
$24×0.3=$ $24×0.03=$ $5×7=$
$0.5×0.7=$ $0.05×0.7=$ $0.05×7=$
$0.3×0.4=$ $0.3×4=$ $0.03×0.4=$
$0.7×0.9=$ $0.8×0.5=$ $0.06×0.5=$

24×3=72
2.4×3=7.2
0.24×3=0.72
24×0.3=7.2
24×0.03=0.72
5×7=35
0.5×0.7=0.35
0.05×0.7=0.035
0.05×7=0.35
0.3×0.4=0.12
0.3×4=1.2
0.03×0.4=0.012
0.7×0.9=0.63
0.8×0.5=0.4
0.06×0.5=0.03

乘风破浪
$8×13=$ $0.8×13=$ $0.08×13=$
$0.8×1.3=$ $0.08×1.3=$ $0.08×0.13=$
$26×2=$ $26×0.2=$ $2.6×0.2=$
$0.26×0.2=$ $0.26×0.02=$ $12×16=$
$1.2×16=$ $1.2×1.6=$ $0.12×1.6=$
$0.12×0.16=$ $13×5=$ $1.3×5=$
$0.03×0.38=$ $0.3×0.38=$ $3×0.38=$
$3×3.8=$ $3×38=$ $0.09×0.12=$

$8×13=104$
$0.8×13=10.4$
$0.08×13=1.04$
$0.8×1.3=1.04$
$0.08×1.3=0.104$
$0.08×0.13=0.0104$
$26×2=52$
$26×0.2=5.2$
$2.6×0.2=0.52$
$0.26×0.2=0.052$
$0.26×0.02=0.0052$
$12×16=192$
$1.2×16=19.2$
$1.2×1.6=1.92$
$0.12×1.6=0.192$
$0.12×0.16=0.0192$
$13×5=65$
$1.3×5=6.5$
$0.03×0.38=0.0114$
$0.3×0.38=0.114$
$3×0.38=1.14$
$3×3.8=11.4$
$3×38=114$
$0.09×0.12=0.0108$

圆满返航
下面的算式中，得数小于$0.5$的有()(填序号)。
① $0.99×0.49$
② $0.49×1.51$
③ $0.51×1.01$
④ $0.89×0.51$
⑤ $0.05×10.1$
⑥ $9.9×0.049$
秘籍点拨 一个数乘比1小(比1大)的数，积一定比这个数小(大)。

①0.99×0.49＜0.5×0.5 = 0.25（估算取两因数近似值） ，实际值更小（因两因数都小于近似值中对应数），准确计算0.99×0.49 = 0.4851＜0.5；②0.49×1.51＞0.49×1 = 0.49且0.49×1.51＞0.5；③0.51×1.01＞0.51＞0.5；④0.89×0.51＜1×0.51 = 0.51，准确计算0.89×0.51 = 0.4539＜0.5；⑤0.05×10.1 = 0.505＞0.5；⑥9.9×0.049＜10×0.05 = 0.5，准确计算9.9×0.049 = 0.4851＜0.5。得数小于0.5的有①④⑥。