方程①x-(10x+1)/6=(2x+1)/4-1②x/0.25-(x+1)/0.4=(x-5)/0.2-2③x-1/2[x-1/2(x-1)-1]=2/3(x-1)+1简答题已知方程3(x-k)+5(x+k)=20有正整数解,且k为正整数,求k的值