A small atomic bomb releases energy equivalent to the detonation of 20,000 tons of TNT; a ton of TNT releases $4 \times 10^{9} \mathrm{J}$ of energy when exploded. Using $2 \times 10^{13} \mathrm{J} / \mathrm{mol}$ as the energy released by fission of $^{235} \mathrm{U},$ approximately what mass of $^{235} \mathrm{U}$ undergoes fission in this atomic bomb?