7个
设有x个
则第一个人买了A=0.5x+0.5(个)
第二个人买了B=0.5*[x-(0.5x+0.5)]+0.5=0.25x+0.25(个)
第三个人买了C=0.5*[x-(0.5x+0.5)-(0.25x+0.25)]+0.5=0.125x+0.125(个)
则A+B+C=x
所以0.5x+0.5+0.25x+0.25+0.125x+0.125=x
解之得:x=7
7个
设有x个
则第一个人买了A=0.5x+0.5(个)
第二个人买了B=0.5*[x-(0.5x+0.5)]+0.5=0.25x+0.25(个)
第三个人买了C=0.5*[x-(0.5x+0.5)-(0.25x+0.25)]+0.5=0.125x+0.125(个)
则A+B+C=x
所以0.5x+0.5+0.25x+0.25+0.125x+0.125=x
解之得:x=7