What we can do is divide this question into two scenarios, keep in mind they need to end in David picking a star from Box B- so it narrows things down.

We can now visualize that the probability he picks a inserts a star is 2/3 and then picks a star would be 3/4.

We can permute this total scenario to be: \frac{2}{3} \cdot \frac{3}{4} = \frac{1}{2}

Our final step is to add these probabilities to formulate the possibility of him picking a star in general, which means to add these two scenarios together: