Answer: 7.8 days
<u>Step-by-step explanation:</u>
Painter can get the job done in 15 days so gets of the job done in 1 day.
Coworker can get the job done in 10 days so gets of the job done in 1 day.
Together, they get of the job done in 1 day.
Painter worked for 3 days so completed of the job.
That leaves a remaining of of the job to be completed.
Let x represent the number of days it will take them to work together.
Painter + Coworker = Together
Multiply by 30 to eliminate the denominator:
Simplify and solve for x:
2x + 3x = 24
5x = 24
x = 4.8
Remember that Painter worked 3 days alone in addition to the 4.8 days they worked together.
So the total time to paint the building is 3 + 4.8 = 7.8