\textbf{167.} A piece of ice at $-20°C$ is placed inside 250 grams of water at $20°C$. After thermal equilibrium is established, 50 grams of ice remains unmelted. What was the initial mass of the ice?
\[
\left(C_{\text{water}} = 4.2\ \frac{\text{J}}{\text{g.K}},\quad C_{\text{ice}} = 2.1\ \frac{\text{J}}{\text{g.K}},\quad L_f = 336\ \frac{\text{J}}{\text{g}},\quad \text{heat exchange is only between water and ice.}\right)
\]
\[
(1)\ 50 \qquad (2)\ 100 \qquad (3)\ 250 \qquad (4)\ 300
\]