Let $a,$ $b,$ $c,$ $d$ be the roots of
\[2x^4 - 8x^3 + 7 = 0.\]Find $a + b + c + d.$
I recognize that the sum of the roots of a polynomial equation is related to the coefficient of the second-highest degree term, by Vieta's formulas. In this case, the second-highest degree term is $-8x^3,$ so the sum of the roots is $-\frac{-8}{2} = 4.$ 