3. 求下面各比的比值。
$5:\frac {2}{9}$ $0.5m^{2}:100dm^{2}$ $0.25:0.75$
$75\% :\frac {1}{5}$ $300m^{2}:0.06$公顷 $4:0.08$
$2.1:0.35$ $0.3$吨：$150kg$ $140:35$
$\frac {7}{6}:3$ 80分：$1.5$时 $2.8:2.7$

$1. 5:\frac{2}{9}=5÷\frac{2}{9}=5×\frac{9}{2}=\frac{45}{2}=22.5；$
2. 因为$1m^{2} = 100dm^{2}，$所以$0.5m^{2}=0.5×100 = 50dm^{2}，$则$0.5m^{2}:100dm^{2}=50:100 = 50÷100 = 0.5；$
$3. 0.25:0.75 = 0.25÷0.75=\frac{0.25}{0.75}=\frac{1}{3}；$
$4. 75\%:\frac{1}{5}=\frac{3}{4}÷\frac{1}{5}=\frac{3}{4}×5=\frac{15}{4}=3.75；$
5. 因为1公顷$ = 10000m^{2}，$所以0.06公顷$=0.06×10000 = 600m^{2}，$则$300m^{2}:0.06$公顷=300:600 = 300÷600 = 0.5；
6. 4:0.08 = 4÷0.08 = 50；
7. 2.1:0.35 = 2.1÷0.35 = 6；
8. 因为1吨 = 1000kg，所以0.3吨=0.3×1000 = 300kg，则0.3吨:150kg = 300:150 = 300÷150 = 2；
9. 140:35 = 140÷35 = 4；
$10. \frac{7}{6}:3=\frac{7}{6}÷3=\frac{7}{6}×\frac{1}{3}=\frac{7}{18}；$
11. 因为1时 = 60分，所以1.5时=1.5×60 = 90分，则80分:1.5时$=80:90 = 80÷90=\frac{8}{9}；$
$12. 2.8:2.7 = 2.8÷2.7=\frac{28}{27}。$

4. 解方程。
$x:15= \frac {4}{5}$ $2x-1.5x= 0.15$
$x+\frac {1}{4}= 50\%$ $x-32\% x= 149.6$
$60\% x+12\% x= 10.8$ $15-25\% x= 7.5$

1.
$x:15 = \frac{4}{5}$
解：$x = 15×\frac{4}{5}$
x = 12
2.
2x - 1.5x = 0.15
解：0.5x = 0.15
x = 0.15÷0.5
x = 0.3
3.
$x+\frac{1}{4}=50\%$
解：x + 0.25 = 0.5
x = 0.5 - 0.25
x = 0.25
4.
x - 32\%x = 149.6
解：0.68x = 149.6
x = 149.6÷0.68
x = 220
5.
60\%x + 12\%x = 10.8
解：0.6x+0.12x = 10.8
0.72x = 10.8
x = 10.8÷0.72
x = 15
6.
15 - 25\%x = 7.5
解：0.25x = 15 - 7.5
0.25x = 7.5
x = 7.5÷0.25
x = 30