How many solutions are there to the equation $$x_1 + x_2 + x_3 + x_4 + x_5 + x_6 = 29$$ where $x_i , i = 1, 2, 3,4,5, 6$ are nonnegative integers such that
a) $x_i > 1$ for $i = 1, 2, 3, 4, 5, 6$?
c) $x_1 \le 5$?
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For (a) replace $x_k$ by $y_k=x_k-2$; then you’re looking for solutions in non-negative integers to
a standard stars and bars problem. For (c) count the solutions in non-negative integers to
and subtract those that have $x_1\ge 6$; you can count those in much the same way that I suggested solving (a).