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

$\dfrac{1}{21}+\dfrac{1}{28}=\dfrac{1}{x}\\ \dfrac{4}{84}+\dfrac{3}{84}=\dfrac{1}{x}\\ \dfrac{7}{84}=\dfrac{1}{x}\\ 7x=84\\ x=12$

Answer 2

Answer:

12 minutes

Step-by-step explanation: