Question

3r/4= 1/8 what's the value of r

Answer 1

Multiplying both sides by 8, we see that 6r=1.  Dividing by 6, r=1/6.

Answer 2

$\frac{3r}{4} =\frac{1}{8}$

Cross multiply.
Note: $\sf\frac{a}{b} = \frac{c}{d} \to ad=bc$

Cross multiplying we get:

$3r(8) = 4(1) \\ 24r = 4\\r = \frac{4}{24} \\\\r=\frac{1}{6}$

I hope that helps :)