Each A atom is adjacent to 3 B atoms. What is the A-C-B bond angle?

Using the simulation to build a system with 5 bonds. The resulting structure is called the Trigonal bipyramidal structure. The two different sites in a trigonal bipyramid are labeled as A and B in the drawing to the right.

In this case, therefore, the bond angle A-C-B is 90 Degrees (right angle)