Given that,
$$a : b = 5 : 3$$
$$\dfrac{a}{b}=\dfrac{5}{3}$$
So,
$$a=5k$$ and $$b=3k$$
Now,
$$\dfrac{(8a — 5b)}{(8a + 5b)}=\dfrac{8\times 5k-5\times 3k}{8\times 5k+5\times 3k}$$
$$\Rightarrow \dfrac{(8a — 5b)}{(8a + 5b)}=\dfrac{25k}{55k}$$
$$\Rightarrow \dfrac{(8a — 5b)}{(8a + 5b)}=\dfrac{5}{11}$$
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