Question

8/12=4/f

Answer 1

well 8 --> 4, we divided by 2.

So 12 ---> what?

Answer 2

8 ---> 8/2 = 4
12 ---> 12/2 = f

Answer 3

Or put another way, these two fractions are equal if there is a number k such that
\[ \frac{8}{12} = \frac{4k}{fk} \]
Hence 4k = 8, k = 2.

So 12 = fk = 2f. Thus f = 6.

I.e.,
\[ \frac{8}{12} = \frac{4}{6} \]

because
\[ \frac{4}{6} = \frac{2 \times 4}{2 \times 6} = \frac{8}{12} \]