A multimode fiber with a 50-m core diameter is designed to limit the intermodal dispersion to 10 ns/km. What is the numerical aperture of this

A multimode fiber with a 50-μm core diameter is designed to limit the intermodal dispersion to 10 ns/km. What is the numerical aperture of this fiber? What is the limiting bit rate for transmission over 10 km at 0.88 μm? Use 1.45 for the refractive index of the cladding.