# Radon-220 has a half-life of 54 seconds. How much of a 20g sample of this gas would remain after 162 seconds?

$2.5 g$ would remain.
$1 \text{half life} = 54 s$
$\frac{162}{54} = 3 \text{half lives} = n$
$20 \frac{g}{2} ^ n = 2.5 g$