10. 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)? 0 The rst and last symbols of the string are distinct digits (which may appear else- where in the string). 0 Precisely four of the symbols in the string are the letter 't'. 0 Precisely three characters in the string are elements of the set V = {51, 6,1', 0, u} and these characters are all distinct