x-15-1/5(x-15)=15+1/5(x-15)(x-15)-1/5(x-15)=15+1/5(x-15)4/5(x-15)=15+1/5(x-15)3/5(x-15)=15方程两边同乘以5/3得x-15=25x=4015*3/5*21+15*4/7*21=3*3*21+15*4*3=3*3*21+3*5*4*3=3*3*(21+20)=9*41=369