Answer
71 grams.
Work Step by Step
For boiling to occur , the pressure inside the cooker is the saturated vapor pressure of water at $120^{\circ}$. We find that value in Table 13–3.
The mass barely stays in place, so its weight equals the force exerted by the gauge pressure inside the pressure cooker.
$$mg=F_{upward}=(P_{inside}-P_{outside})A$$
$$mg=(P_{inside}-P_{outside})\pi r^2$$
$$m=\frac{(P_{inside}-P_{outside})\pi r^2}{g}$$
$$m=\frac{(1.99\times10^{5}Pa-1.01\times10^{5}Pa)\pi (0.0015m)^2}{9.80m/s^2}$$
$$m=0.071kg$$
