The Boyle Law is $\"\"$ , where $\"\"$ is a constant.

At certain instant, volume $\"\"$ and the pressure $\"\"$ .

Rate of change of pressure $\"\"$ .

Consider $\"\"$ .

Differentiate on each side with respect to $\"\"$ .

$\"\"$

Substitute $\"\"$ , $\"\"$ and $\"\"$ in the above equation.

$\"\"$

Here negative sign indicates that the volume is decreasing.

Volume is decreasing at a rate of 80 cm³/min.