  Quasi-periodic oscillations (QPOs) observed in a giant flare of a strongly
magnetized neutron star (magnetar), are carrying crucial information for
extracting the neutron star properties. The aim of the study is to constrain
the mass and radius of the neutron star model for GRB 200415A, by identifying
the observed QPOs with the crustal torsional oscillations together with the
experimental constraints on the nuclear matter properties. The frequencies of
the crustal torsional oscillations are determined by solving the eigenvalue
problem with the Cowling approximation, assuming a magnetic field of about
$10^{15}$G. We find that the observed QPOs can be identified with several
overtones of crustal oscillations, for carefully selected combinations of the
nuclear saturation parameters. Thus, we can inversely constrain the neutron
star mass and radius for GRB 200415A by comparing them to the values of nuclear
saturation parameters obtained from terrestrial experiments. We impose further
constraints on the neutron star mass and radius while the candidate neutron
star models are consistent with the constraints obtained from other available
astronomical and experimental observations.
