Since he checked 30 seats, there must have been a total of 10 unchecked seats, making the number of total seats 40. 5 seats a building means eight buildings!
8! Am I right?
You're taking a stroll around some city streets and suddenly you remember that one of your friends works in the area but you're not sure where. All the buildings have windows and you want to see if you can spot them.
All the offices in the area have the same layout of 4 windows and 5 seats/work spaces:
The 4 seats that are next to a window can only be viewed from the window that they are by and the middle seat cannot be seen from any window.
You realise that your friend could be sitting in any one of the seats in any one of the many buildings in the area so you decide to look around for a while and look through 30 windows in total.
You don't see them but estimate that the chance you had of spotting your friend (regarding the information you knew) was about 75%, but how many buildings would there have to have been for your estimation to be correct?