Решаем пропорциями...
18(2/3+x)=14(3/2+x)
раскрываем скобки:
12+18x=21+14x
18x-14x=21-12
4x=9
x=9:4
x=2 1/4
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(5x-12):3/8= 12,5 : 1 9/16
1 9/16(5x-12)=12,5×3/8
125/16x-75/4=75/16
125/16x=75/16+75/4
125/16x=75/16+300/16
125/16x=375/16
x=375/16×16/125
x=375/125
x=3