0.3:2/5=3 :4比值=0.75
2/5=0.4，原题变为0.3:0.4=3:4
0.3：2/5=3/10:2/5=3/10:4/10=3:4=3/4
=（0.3×10）∶（2/5x10）=3：4=3/4
=3/10 : 2/5=3/10 * 5/2=3/2 *1/2=3/4
