1.3/10x+7x-1/4x=21.15 (0.3+7-0.25)x=21.5 7.05x=21.5 x=21.5/7.05=3.04962.5（x-4)+3(9-x)=12+7(7-x) 5x-20+27-3x=12+49-7x (5-3+7)x=12+49-27+20 x=63.(19x+2/5)÷2=4 19x+2/5=8 19x=8-0.4=7.6 x=7.6/19=0.44.(8/...第五题我怎么有些看不懂？能重新写一遍吗？x+1/4(1-2/7x)=7/8展开：x+1/4-1/14x=7/8 移项合并：（1-1/14）x=7/8-1/413/14x=5/8 两端同除以13/14:x=5*14/(8*13)=35/52