Sam can clean the pool in 5 hours; his coworker Glenn can clean the same pool in 7 hours. One day Sam started to clean the pool by himself and worked for an hour. Then Glenn joined him, and the finished
cleaning the pool. How long did it take them to finish cleaning the pool?

23/12 hours or 1.9167 hours

Step-by-step explanation:

We are told that Sam can clean the pool in 5 hours. This implies 1:5

Also, his coworker Glenn can clean the same pool in 7 hours. This implies 1:7

Since it's the same pool, let's generate a workrate formula.

Thus;

(1/5)x + (1/7)x = 1

Where x is the amount of time used by both of them to clean 1 pool

Simplifying it, we multiply each term by 35 to get;

7x + 5x = 35

12x = 35

x = 35/12

We are now told that One day Sam

started to clean the pool by himself and worked for an hour.

Thus, remaining time after Sam worked 1 hour is; (35/12) - 1 = 23/12 hours

Thus, time they both used to finish cleaning the pool is 23/12 hours

a. is correct