In triangle $XYZ$, angle $Z$ is a right angle. Point $A$ lies on $XY$, point $B$ lies on $YZ$, and $AB$ is parallel to $XZ$. If the length of $XZ$ is 40 units, the length of $AB$ is 15 units, and the area of triangle $XYZ$ is 700 square units, what is the length of $BZ$, in units?