A piece of ice (heat capacity = 2100 J $$kg^{-1} {^\circ}C^{-1}$$ andlatent heat = $$3.36 \times 10^5 J kg^{-1}$$) of mass m grams is at $$-5 ^\circ C$$ 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 1 gmof ice has melted. Assuming there is no other heat exchange in the process, the value of m is