Question 283900
This belongs under rate of work, painting  and pool filling.
Look for some example in that category. There have been several in the last few days.
You can see now  why it is important to use the right category.
I will set it up since John's way is much too complicated.
x/3-x/5=1
5x-3x=15
2x=15
x=7.5 hours