a) Consider the equation $x_1 + x_2 + x_3 + x_4 = 35$. How many different solutions does this equation have if all the variables must be positive integers? Enter the exact numeric answer.
b) Suppose that a license plate consists of three letters followed by three digits. How many different license plates start with the letter A if letters and digits cannot be repeated? Enter the exact numeric answer.
A_ _ _ _ _ . The last three number should not be repeated, so $P(10,3)$ and then the letters can be chosen randomly so $C(26,2)$