the number of positive integer solutions for the equation x + y + z + t = 21 is
"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 . 21 - 1 c 4 - 1 = 20 c 3 = 1140 answer : a"
divide(factorial(subtract(21, const_1)), multiply(factorial(subtract(const_4, const_1)), factorial(subtract(subtract(21, const_1), subtract(const_4, const_1)))))