Which equation can be used to solve for the rate, r, for sam to change all the light bulbs in the office building by himself?

Step-by-step explanation:

The complete question is

Working together, sam and charlie can change all the light bulbs in the office building in 3 hours. It would have taken charlie 8 hours to do the job alone. Which equation can be used to solve for the rate, r, for sam to change all the light bulbs in the office building by himself?

The finished work is represented by 1, which means the 100% of all.

Also, the ratio for charlie to do the job alone is

, which means that he take 8 hours by himself, and 3 hours together.

The ratio for sam to change all the light bulbs is

, which means that he and charlie take 3 hours to do all the job, and represents the time is can do the job alone.

The sum of these ratios must be equal to 1, as we said before, represents 100% of the job done.

Therefore, the final expression is