If a question asked: How many integer solutions of $x_1+x_2+x_3+x_4=30$ with $-9 \leq x_i \leq 21$? How would this be solved?
I understand how to solve if the inequality was $0 \leq x_i \leq 21$?, but how to solve between a negative and positive inequality?
$N = \binom{30 + 4-1}{30}$
$N(A_i) = \binom{(30 - ?) +4-1}{30 - ?}$
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