Question

Two women painted a barn. Barbara can paint it alone in 5 days, and Sara in 8 days. They

started to paint it together, but after 2 days, Sara got bored and Barbara finished alone. How
long did it take her to finish?

Answer 1

Answer: she will need 2.8 days to finish the work.

Explanation:

Let's define:

D = number of days.

B = rate at which Barbara paints a barn

S = rate at which Sara paints a barn

We know that Barbara can paint a barn in 5 days, then:

B*5 days = 1 barn

B = 1/5 barns per day.

And we know that Sara can paint it in 8 days, then:

S*8 days = 1 barn

S = 1/8 barns per day.

Now, when they work together, their rates are added.

Then if they work together for two days, they paint a total of:

(1/5 barns per day + 1/8 barns per day)*2 days

(8/40 barns per day + 5/40 barns per day)*2 days

(13/40 barns per day)*days = 26/40 barns

Now Sara needs to paint he other 14/40 barns in her own, she will need D days, then:

(1/8 barns per day)*D = 14/40 barns

D = (14/40 barns)/(1/8 barns per day) = (14*8/40) = 2.8 days.

So she will need 2.8 days to finish the work.