(a) E = mc^2, where E = 18 * 10^12 J and c = 3 * 10^8 m/s.  Thus,

    m = E/c^2 = (18 * 10^12 kg m^2/s^2) / (3 * 10^8 m/s)^2
      = (18 * 10^12)/(9 * 10^16) = 0.0002 kg

    This is 0.2 gram of matter, or about one-twelfth the mass of a penny.

(b) The fraction of fissioned uranium that was converted to energy was

    0.0002/1.000 = 0.0002, or about one-fiftieth of one percent.