The number of positive integer solutions for the equation x +y +z + t = 10 is  Pick: A) 60 B) 74 C) 84 D) 54 E) 104

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 . 10 - 1 c 4 - 1 = 9 c 3 = 84 answer : c"
Answer: C