If the moment of inertia of a disk kept in xz plane about x and y axis are, respectively, 30 kgm² and 40 kgm², then moment of inertia about z-axis (in kgm²) will be

According to the perpendicular axis theorem,
$I_x+I_z=I_y$
Where I_x and I_z are the moment of inertia of planer object about the x and z axis respectively, which lie in the same plane of the body. I_y is moment of inertia of the object about y axis which is perpendicular to the plane of the body.
According to the given question:

$\begin{array}{l}30+{\mathrm{I}}_{\mathrm{z}}=40\\ \Rightarrow {\mathrm{I}}_{\mathrm{z}}=10{\mathrm{kgm}}^2\end{array}$