This will be a simulation of coral mine.
In order to make it interesting and not just some spam post:
Every user has to think of some mathematical problem or physics exercise and other one has to solve it quoting a new math or physics problem.
Moreover, try to find more difficult problems/exercises than it was before and try to solve it using your brains not a computer :P
Remember, here you getting 100 corals for every post you make, here, it comes a challenge with it.
Rules simple:
1. Do not post anything here, except the solution of the last quote and new math/physics exercise.
2. If the answer was right, author of the problem has to tip 200 corals to a user that solved it. So it is also an encouragement to make difficult exercises :P
3. Users that will be participating, once in a while, will randomly receive random sums of corals (from 100 to 1k) from Sour_Candy.
SO, LET'S MAKE SOME CORALS FELLAS: Four gentlemen have put off four identical hats in the locker room. What is the probability that on leaving at least one of them will get back his own hat ?