If a job is complete by a and b in z days,b and c in x days and a and c in y days. In how many days that a,b and c can be work simultaneously?
Choose one:  A) 2(xyz/xy+yz+xz)  B) xyz/xy+yz+xz  C) (xy+yz+xz)/xyz  D) cannot be determined  E) 2(xy+yz+xz)/xyz
Option A is answer.   one day work of a+b,b+c and c+a is 1/z,1/x and 1/y.   That is , 2(a+b+c)=(yz+xz+xy)/xyz.   a+b+c=(yz+xz+xy)/2xyz.(one day work).   Total work done by a,b and c is 2xyz/(xy+yz+xz).
<result>A</result>
If a job is complete by a and b in z days,b and c in x days and a and c in y days. In how many days that a,b and c can be work simultaneously?