20×(12x-10)/(21×20)+21×(7x-9)/(20×21)=14×(2-x)/(15×14)+15×(8x-9)/(14×15)(240x-200)/420+(147x-189)/420=(28-14x)/210+(120x-135)/210(387x-389)/420=(106x-107)/210(387x-389)=2(106x-107)175x=175x=1