设：甲、乙、丙各单独做各需要x、y、z天则：x=a(y+z) y=b(x+z) z=c(x+y)得：a=x/(y+z) b=y/(x+z) c=x/(x+y)a+1=(x+y+z)/(y+z) b+1=(x+y+z)/(x+z) c+1=(x+y+z)/(x+y)则：1/（a+1)+1/(b+1)+1/(c+1)=2
2
