It requires 5 hours for Mary to paint a building. If she and Jack work
together, they can paint the same building in 3 hours. Find the time, in
hours, it would take Jack to the building alone.

7.5 hours

Step-by-step explanation:

Mary's rate of work is 1/5 and combined rate of work is 1/3 hence for Jack, the rate of work will be given by subtracting Mary's rate of work from the total. Therefore, Jack's rate of work is 1/3- 1/5=2/15

The number of hours is usually the reciprocal of the rate of work. Therefore, Jack's number of houra will be reciprocal of 2/15 which is equivalent to 15/2=7.5 hours

Therefore, Jack alone would take 7.5 hours to do the painting.