Person 1 can do a certain job in twenty-one minutes, and person 2 can do the same job in twenty-eight minutes. When completing the job together, which expression would be used to represent the amount of work done by person 1?

Question

anonymous · Answer

21
1/21
x/21
21x

anonymous · Answer

$$
\frac 1{21}+ \frac 1{20}=\frac 1 x

$$
1/21 is the portion of the job done by Person 1 in one minute. 
1/20 is the portion of the job done by Person 2 in one minute. 
1/x  is the portion of the job done by  in one both in one minute. 

I think the problem is not worded clearly