Question

One person can do a certain job in twenty-one minutes, and another person can do the same job in twenty-eight minutes. How many minutes will it take them to do the job together?
6 min
12 min
24 min

Answer 1

the correct answer is 12

Answer 2

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 :-

$\frac{1}{T}=\frac{1}{21}+\frac{1}{28}\\\\\Rightarrow\frac{1}{T}=\frac{4+3}{84}\\\\\Rightarrow\frac{1}{T}=\frac{7}{84}\\\\\Rightarrow\frac{1}{T}=\frac{1}{12}\\\\\Rightarrow T=12$

Hence, they will take 12 minutes to do the job together.