On a particular day, seven persons pick six different books, one each, from different counters at a public library. At the closing time, they arbitrarily put their books at vacant counters. The probability that at least four books are at their previous places is

On a particular day, seven persons pick six different books, one each,

1.	On a particular day, seven persons pick six different books, one each, from different counters at a public library. At the closing time, they arbitrarily put their books at vacant counters. The probability that at least four books are at their previous places is
Answer» On a particular day, seven persons pick six different books, one each, from different counters at a public library. At the closing time, they arbitrarily put their books at vacant counters. The probability that at least four books are at their previous places is