Ice at $-20°C$ is added to 50g of water at $40°C$ . When the temperature of the mixture reaches $0°C$ , it is found that 20g of ice still unmelted. The amount of ice added to the water was close to 
(Take, specific heat of water = $4.2J/g/°C$ , specific heat of ice =$2.1J/g/°C$ and latent heat of fusion of water at $0°C=334J/g$  )