Answer
$A=20~millimeters~of~mercury$
It represents the variation of the pressure around 100 millimeters of mercury. That is, the pressure goes from 80 to 120 millimeters of mercury.
Work Step by Step
In the function: $y=y_0+A~cos~Bt$, $|A|$ is the amplitude.
In $P=100-20~cos~\frac{8\pi t}{3}$ the amplitude is:
$A=|-20|=20$
