What is the number of integer solutions of the following equation ? Find the solution without computing 9 combinations. The equation is:
$$x_1+x_2+x_3+x_4+x_5+x_6<10$$
with $x_i\geq 0$ for $i=1,2,\dots,6$.


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*What is the number of integer solutions of the following equation ?

*Find the solution without computing 9 combinations.


Write 
$$x_1+x_2+...+x_6=9-y$$
with $y \ge 0$ and solve
$$x_1+x_2+...+x_6+y=9$$
for integer solution.
