A Styrofoam bucket of negligible mass contains 1.75 kg of water and 0.455 kg of ice. More ice, from a refrigerator at -16.0 °C, is added to the mixture in the bucket, and when thermal equilibrium has been reached, the total mass of ice in the bucket is 0.768 kg. Assuming no heat exchange with the surroundings, what mass of ice was added?