17x-12x=35 解:5x=3.5 x=0.7 21x-2.7×4=8.1 解:21x-10.8=8.1 21x=18.9 x=0.9 8x+4=40 8x+4-4=40-4 8x=36 8x÷8=36÷8 x=4.5 7(x-10)=140 7(x-10)÷7=140÷7 x-10=20 x-10+10=20+10 x=30 5x-2×0.8=33.4 解:5x=33.4+1.6 x=35÷5 x=7