To find out how many liters of water will be in the tank after 5 minutes, you can substitute \( t = 5 \) into the expression \( 300 + (-3t) \).
Calculating this, we have:
\[ 300 + (-3 \times 5) = 300 - 15 = 285 \]
Therefore, after 5 minutes, there will be 285 liters of water in the tank.