If ∆ABC ∼ ∆DEF such that 2 AB = DE and BC = 6 cm, find EF.

$\because \triangle A B C \sim \triangle D E F$

$\therefore \frac{A B}{D E}=\frac{B C}{E F}$

$\Rightarrow \frac{1}{2}=\frac{6}{E F}$

$\Rightarrow E F=12 \mathrm{~cm}$