Simple word problem, but I am confused:
"It takes me $12$ hours to read a book at normal pace; if I read it $\mathbf2$ times as fast, how many hours does it take?"
The given answer is $4$ hours, but I thought it would be $6$ hours since $$ \frac{12\text{ hrs}}2 = 6\text{ hrs}\,?$$
You are correct.
Had the question instead been "It takes me 12 hours to read a book at normal pace; if I read it 2 times faster, how many hours does it take?", then I would interpret the hypothetical speed to be 3 times as fast as the normal pace, in which case the corresponding duration is $$\frac{12\text{ hrs}}3=4\text{ hrs}.$$