At any given time, the probability that there are more than two hundred books checked out from the school...

At any given time, the probability that there are more than two hundred books checked out from the school library is 1/2. The probability that there are more than two hundred books checked out from the school library and at least twenty of those books are history books is 1/5.
What is the probability that at least twenty history books are checked out from the school library given that more than two hundred books are checked out?
Write your answer as a fraction or decimal. Do not round.