望采纳,谢谢
×0.25=1/4,25=1/4,所以这道题就等于1/4×(13/31+18/31+1)也就等于1/4×2结果就等于1/2
0.25×13/31+18/31×25%+1/4=0.25×(13/31+18/31)+1/4=0.25×1+0.25=0.5
0.25×13/31+18/31×25%+1/4=1/4×13/31+18/31×1/4+1/4=1/4×(13/31+18/31)+1/4=1/4×1+1/4=1/4+1/4=2/4=1/2