The number of positive integer solutions for the equation x +y +z + t = 18 is Choose one.
A) 640  B) 780  C) 380  D) 540  E) 680

Rationale: "the number of positive integer solutions for the equatio fx 1 + x 2 + ⋯ + xn = k ( k - 1 ) c ( n - 1 ) - where k is the number and n is number of variable in the equation . 18 - 1 c 4 - 1 = 17 c 3 = 680 answer : e"
Answer: E