Answer:
In series:
[tex] \frac{1}{r} = \frac{1}{r_{1} } + \frac{1}{r _{2} } + \frac{1}{r_{3} } [/tex]
[tex] \frac{1}{r} = \frac{1}{4} + \frac{1}{6} + \frac{1}{8} \\ \\ \frac{1}{r} = \frac{13}{24} \\ \\ r = \frac{24}{13} \\ { \underline{r = 1.85 \: Ω}}[/tex]
In parallel:
[tex]r = r _{1} + r _{2} + r _{3} \\ r = 4 + 6 + 8 \\ r = 18Ω[/tex]
