There are 7 books all in all. Since the order is important, there are 7P2 (permutation of 7 taken 2) ways in which Tena can do the repost. If she picks historical books first then followed by the science-fiction, there are 3 x 4 = 12 ways. The probability is therefore equal to 12/(7P2) which is equal to 2/7. The answer is letter c.
