Zooplus @ Devoxx

На стенде местной фирмы давали решать задачи.

☆ - за правильное решение выдавали сумку с барахлом:

In a 3 x 3 grid, use the digits 1 through 9 to fill in the cells so each horizontal, vertical, and diagonal row of three has a sum of 15. No digit may be used more than once.

☆☆ - за правильное решение выдавали чашку:

There are 100 attendees joining a Devoxx lecture. Each of them a sit assigned to them. Unfortunately the first attendant forgot which place was his, so he picked a random one. Each of the remaining attendees gets into the room and take their seat unless it's busy. In that case, they pick a random one.
What is the chance that the last attendee (number 100) will get his seat?

☆☆☆ - за правильное решение выдавали футболку:

This meeting is so boooooring... - gasped one of 29 programmers.
You think this is boring? - replied the meeting organizer Rico - Let's play a game then. Each of you gets a hat. You see each other's hats, but not your own. When I change the slides and you know your hat color you can leave, otherwise you will listen to my talk forever!
That's not fair, you can't know it for sure - said Gregor.
What's with your attitude, you can do it! - replied Rico.
After the first slide everyone with blue hat left, after the second no one left, after the third 8 people left, after the next at least 1 person. Gregor and his friend left later, but not at the same time.
After how many slides, was Rico left alone?