MS Probability question

The principal of a school gives different certificates of award to 5 students
in an assembly. Because of clerical mistake, all certificates are given in a
random order. Find the probability that

a) at least 3 certificates are given correctly.

b) all certificates are given wrongly.

Here are the answers of the above questions, but I don't know how to
calculate it. Plx help me to solve this question step by step with
explanation. a) 11/120 b)11/30 THX><