Let x be the set of the 26 lowercase english letters and 10 decimal digits. how many x-strings of length 15 satisfy all of the following properties (at the same time)?

a. the first and last symbols of the string are distinct digits (which may appear elsewhere in the string).
b. precisely four of the symbols in the string are the letter ’t’.
c. precisely three characters in the string are elements of the set v fa; e; i; o; ug and these characters are all distinct.