a)
[tex] \frac{3}{4} = \frac{n}{8} \\ \\ n = \frac{3 \times 8}{4} = 6[/tex]
b)
[tex] \frac{3}{4} = \frac{9}{n} \\ \\ n = \frac{4 \times 9}{3} = 12[/tex]
c)
[tex] \frac{3}{4} = \frac{20}{n} \\ \\ n = \frac{20 \times 4}{3} = \frac{80}{3} [/tex]
d)
[tex] \frac{3}{4} = \frac{2n + 3n}{20} \\ \\ \frac{3}{4} = \frac{5n}{20} \\ \\ \frac{3}{4} = \frac{n}{4} \\ \\ 3 = n \\ n = 3[/tex]