a:c=（a：b）*（b：c）=(3:1.2)*(0.75:1/2)=45:12=15:4c:a=4:15
1.2a = 3/2b ; 0.75c = 1/2b;6/5a = 3*3/4cc:a = 2/5:3/4 = 8:15;
