Pure water cooled to - 15°C is contained in a thermally insulated flask. Some ice is added into the flask. The fraction of water frozen into ice is :

Here if m is the mass of ice and M is the mass of water, then 80 m = m × 0.5 × 15 + (M-m) × 1 × [0-(-15)]= m × 7.5 +15 M - 15mor   mM=635∴Correct choice is (2).