If 3, 6, 12 and b are in continued proportion, then what is the value of b?

3, 6, 12 and b are in continued proportion

If a, b, c, and d are in proportion then 

​$$a : b :: c : d \implies \frac{a}{b} = \frac{c}{d}$$

​$$3 : 6:: 12 : b\\ \rightarrow \frac{3}{6} = \frac{12}{x} \\ \ \\ 3x = 12 \times 6 \\ b = 24$$