Exercise 8.

How many integer solutions to \(x_1 + x_2 + x_3 + x_4 = 25\) are there for which \(x_1 \ge 1\text{,}\) \(x_2 \ge 2\text{,}\) \(x_3 \ge 3\) and \(x_4 \ge 4\text{?}\)

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