To solve for \( y \) after 12 minutes using the equation \( y = 10x + 20 \), we can substitute \( x = 12 \) into the equation:
\[ y = 10(12) + 20 \]
Now, calculate:
\[ y = 120 + 20 \]
So,
\[ y = 140 \]
Therefore, the temperature of the water after 12 minutes is \( y = 140 \) degrees.