The sum of their ages in the future

Let $a$ and $b$ be the present ages of the two friends.

At present:

$\dfrac{a}{b}=\dfrac{2}{1}$

$a=2b$ $\implies$ $\color{#D61F06}\boxed{1}$

Thirty years ago:

$\dfrac{a-30}{b-30}=\dfrac{5}{1}$ $\implies$ $\color{#D61F06}\boxed{2}$

From $\color{#D61F06}\boxed{1}$ in $\color{#D61F06}\boxed{2}$ , we get

$a=80$ and $b=40$

Twenty years form now, the sum of their ages is:

$a+20+b+20=80+20+40+20=$ $\boxed{160}$