If the polynomial $f(x)=4x^4-a.x^3+b.x^2-c.x+5$ where $a,b,c \in \mathbb{R}$ has $4$ positive real zeroes(roots) say $r_1,r_2,r_3, \ and \ r_4$, such that $\frac{r_1}{2}+\frac{r_2}{4}+\frac{r_3}{5}+\frac{r_4}{8}=1$
Find the value of $a$.

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