One reaction gives 163MeV, means one atom of uranium 235 gives 163MeV. 10 kg consist of how many atoms? Take 10kg / 235x1.66x10-27 kg to give number of atoms N.
So N reactions (10kg) gives 163MeV x (10kg/235u)= 4.18x1027 MeV.
If use Na must find number of moles n= 10 kg/235x10-3kg =42.6 moles. Number of atoms N = 42.6x(6.02x1023). Energy released = 42.6x(6.02 x 1023) x 163 MeV= 4.18 x1027 MeV.
Both methods can be used depending on which makes more sense to you.
