Let a Be the Set of All Strings of Decimal

Short Answer

Let A be the set of all strings of decimal digits of length 5. For example 00312 and 19483 are two strings in
A.
You pick a string from A at random. What is the probability that
(a) the string begins with 7575.
(b) the string has no 4 in it.
In questions 13-16 you have 40 different books (20 math books, 15 history books, and 5 geography books).