If $20 x^2+13 x-15$ can be written as $(a x+b)(c x+d)$, where $a, b, c$ and $d$ are integers, what is $a+b+c+d$? 
If $20 x^2+13 x-15$ can be written as $(a x+b)(c x+d)$, where $a, b, c$ and $d$ are integers, what is $a+b+c+d$ ?

The quadratic formula tells me that $x=\dfrac{-13 \pm 20\sqrt{3}}{40}.$
How to proceed?

Another way to realize the solution (through inspection):
\begin{align*}
20x^{2} + 13x - 15 & = (20x^{2} - 12x) + (25x - 15)\\\\
& = 4x(5x - 3) + 5(5x - 3)\\\\
& = (4x + 5)(5x - 3)
\end{align*}
