A bubble chart displays three variables: $X$ on the x-axis, $Y$ on the y-axis, and $Z$ represented by the bubble size. If bubble A has a radius of $r_A$ and represents a value $Z_A = 150$, and bubble B has a radius of $r_B = 3r_A$, what value $Z_B$ does bubble B represent, assuming bubble size (visual area) is directly proportional to the variable $Z$?