Shortcut
$$ab : bc : ca = 3 : 1 : 2$$
c = 2 a = 6 b = 3
$$\frac{a}{bc} : \frac{b}{ca}$$
$$\Rightarrow \frac{6}{6} : \frac{3}{12}$$
$$1 : \frac{1}{4}$$
$$\Rightarrow 4 : 1$$
Explanation:
$$ab : bc :ca = 3 : 1 : 2$$
$$\Rightarrow  \frac{1}{c} : \frac{1}{a} : \frac{1}{b} = 3 : 1 : 2$$
$$c = \frac{k}{3}, a = k, b = \frac{k}{2}$$
Ratio $$c = 2k, a = 6k, b = 3k$$
$$\frac{a}{bc} : \frac{b}{ca}$$
$$\frac{6}{6} : \frac{3}{12}$$
4 : 1
Hence the answer is option B
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