A piece of ice (heat capacity =2100Jkg−1oC−1 and latent heat =3.36×105Jkg−1) of mass m grams is at −5oC 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 gm of ice has melted. Assuming there is no other heat exchange in the process, the value of m is