A $400-$ft long roller conveyor operates at a velocity $=50$ ft$/$min and is used to move parts in containers between load and unload stations. Each container holds $15$ parts. One worker at the load station is able to load parts into containers and place the containers onto the conveyor in $45$ sec$.$ It takes $30$ sec to unload at the unload station. Determine center$-$to$-$center distance $($ft$/$container$)$ between containers.