How many solutions possible for the equation $x_1+x_2+x_3+x_4+x_5=55$ if all $x$ are non-negative integer:
- No restrictions. The solution is $C(55 + 4, 4) = C(59,4)$ but I fail to see why, can someone explain this to me?
- Every $x_k$ is odd.
- If $x_1>=1,x_2>=2,x_3>=3,x_4>=2,x_5>=1,$