The technique known as potassium-argon dating is used to date old lava flows and thus any fossilized skeletons found in them, like this 1.8-million-year-old hominid skull. The potassium isotope ^40K has a 1.28 billion year half-life and is naturally present at very low levels. ^40K decays by beta emission into the stable isotope ^40Ar. Argon is a gas, and there is no argon in flowing lava because the gas escapes. Once the lava solidifies, any argon produced in the decay of ^40K is trapped inside and cannot escape.
What is the age of a piece of solidified lava with a ^40Ar/^40K ratio of 0.31?