Answer: 12 min
Step-by-step explanation:
Given: Time taken by one person to do a job = 21 minutes
Time taken by another person to do a the same job = 28 minutes
If they work together then the total time taken by both of them to complete the job is given by :-
[tex]\frac{1}{T}=\frac{1}{21}+\frac{1}{28}\\\\\Rightarrow\frac{1}{T}=\frac{4+3}{84}\\\\\Rightarrow\frac{1}{T}=\frac{7}{84}\\\\\Rightarrow\frac{1}{T}=\frac{1}{12}\\\\\Rightarrow T=12[/tex]
Hence, they will take 12 minutes to do the job together.
