How many solutions are there to the equation x₁ + x₂ + x₃ + x₄ + x₅ + x₆6 = 25 in which each xi is a non-negative integer and …
1) There are no other restrictions.
2) xi ≥ 3 for i = 1, 2, 3, 4, 5, 6
3) 3 ≤ x1 ≤ 10
4) 3 ≤ x1 ≤ 10 and 2 ≤ x2 ≤ 7