A gun fires a lead bullet of temperature 300 K into a wooden block. The bullet having melting temperature of 600 K penetrates into the block and melts down. If the total heat required for the process is 625 J, then the mass of the bullet is ......... grams.
(Latent heat of fusion of lead $=2.5\times {{10}^{4}}\text{JK}{{\text{g}}^{{-1}}}$ and specific heat capacity of lead $=125\text{JK}{{\text{g}}^{{-1}}}$)