Answer:
She needs 16 drops of blue food coloring.
Explanation:
We are given that,
Drops of yellow food coloring used = 3
Drops of blue food coloring used = 4
So, the ratio of yellow food coloring to the blue food coloring =

It is required to find the drops of blue food coloring if she uses 12 drops of yellow food coloring.
Let the drops of blue food coloring = x.
So, we have,

i.e.

i.e. x = 4 × 4
i.e. x = 16
Thus, she needs 16 drops of blue food coloring.