The water equivalent of a calorimeter is \(10 \mathrm{g}\) and it contains \(50 \mathrm{g}\) of water at \(15^{\circ} \mathrm{C}\). Some amount of ice, initially at \(-10^{\circ} \mathrm{C}\) is dropped in it and half of the ice melts till equilibrium is reached. What was the initial amount of ice that was dropped (when specific heat of ice \(=0.5 \mathrm{cal} \mathrm{gm}^{-1}{ }^{\circ} \mathrm{C}^{-1},\) specific heat of water \(=1.0\)
cal \(\mathrm{gm}^{-1 \circ} \mathrm{C}^{-1}\) and latent heat of melting of ice \(=80\) cal \(\mathrm{gm}^{-1}\) )?

1. A \(10 \mathrm{g}\)
2. B \(18 \mathrm{g}\)
3. C \(20 \mathrm{g}\)
4. D \(30 \mathrm{g}\)