MATL, 8 7 bytes
tsGE-/p ###Explanation
Explanation
Let's use input [3 4 5] as an example
t % Take input implicitly. Duplicate % STACK: [3 4 5], [3 4 5] s % Sum of array % STACK: [3 4 5], 12 G % Push input again % STACK: [3 4 5], 12, [3 4 5] E % Multiply by 2, element-wise % STACK: [3 4 5], 12, [6 8 10] - % Subtract, element-wise % STACK: [3 4 5], [6 4 2] / % Divide, element-wise % STACK: [0.5 1 2.5] p % Product of array. Implicitly display % STACK: 1.25