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

The number of minutes that it will take for them to do the job together is:

• 12 minutes

What is the formula for work done together?

To arrive at the correct answer, it is necessary to factor in the number of minutes worked by each individual. For that, we can assign them х and у.

So the number of hours worked by the person х is 21 minutes

and the number of hours worked by person у is 28 minutes

When they work together, they will spend:

$XY minutes / X+ Y minutes$

$21 min * 28 min = 588 minutes\\\\21 min + 28 min = 49 minutes$

Therefore;

$588 / 49 = 12 minutes.$

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