甲占总数 1/2÷(1+1/2)=1/3
乙 1/3÷(1+1/3)=1/4
丙 1/4÷(1+1/4)=1/5
总数 260÷(1-1/3-1/4-1/5)=260÷(1-20/60-15/60-12/60)=1200
甲 1200x1/3=400
乙 1200x1/4=300
丙 1200x1/5=240
甲占总数 1/2÷(1+1/2)=1/3
乙 1/3÷(1+1/3)=1/4
丙 1/4÷(1+1/4)=1/5
总数 260÷(1-1/3-1/4-1/5)=260÷(1-20/60-15/60-12/60)=1200
甲 1200x1/3=400
乙 1200x1/4=300
丙 1200x1/5=240