A machine makes two components; call them types A and B$.$ It takes $630$ seconds to switch production between the component types. During that time, no production occurs. When in production, each unit of A or B requires $0\mathrm{.}5$ second to be completed. The two components, A and B$,$ are combined in an assembly process to make a final product; call it C$.$ The assembly step can combine the two components into $1$ unit every $4$ seconds, or $15$ units per minute.

Suppose the machine rotates between one batch of $2,520$ units of A and $2,520$ units of B$.$ What is the average inventory of B components?

Average inventory: $\_\_\_$ units