To find how long it will take for the rock to reach the river, we can set the height \( h \) to 0 in the given equation:
\[ h = -16t^2 + 320 \]
Setting \( h = 0 \):
\[ 0 = -16t^2 + 320 \]
Rearranging the equation:
\[ 16t^2 = 320 \]
Dividing both sides by 16:
\[ t^2 = 20 \]
Taking the square root of both sides:
\[ t = \sqrt{20} \] \[ t = \sqrt{4 \times 5} = 2\sqrt{5} \]
Calculating \( \sqrt{5} \) gives approximately 2.236, so:
\[ t \approx 2 \times 2.236 \approx 4.472 \]
Therefore, the time \( t \) it will take for the rock to reach the river is approximately 4.5 seconds.
Thus, the answer is:
4.5 seconds.