3 1/2 : 5/8 = 7/2:5/8 = 7/2 × 8/5 = 28/5
4升:50毫升 = 4000毫升 :50毫升 = 80:1
7x/9+3=9.3
7x/9=6.3
7x=56.7
x=8.1
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