IIT JEE Advanced 2010 Paper 1

A piece of ice (heat capacity = 2100Jkg−1°C−1 and latent heat = 3.36×105Jkg−1) of mass m grams is at –5°C at atmospheric pressure.
It is given 420 J of heat so that the ice starts melting. Finally, when the ice–water mixture is in equilibrium, it is found that 1g of ice has melted. Assuming there is no other heat exchange in the process, the value of m is _____.