说明这个数加上1/7等于7/9x45%，列式为7/9x45%-1/7，算出来是29/140
7/9x45%-1/7=39/70
7/9×45％-1/7
x+1/7=7/9*0.45→x=7/9*9/20-1/7=7/20-1/7=9/140
