Cesium-137 has a half-life of 30 years. If a 2.0 g sample of cesium-137 decays over 120 years, how much would remain?

Since cesium-137 has a half-life of 30 years, this means that half of the sample will disappear every 30 years.
Over a period of 120 years, cesium-137 will undergo 4 half-lives So, to get the final answer, we divide 2.0g in half four times:
30 years: \(2 \div 2 = 1\)
60 years: \(1 \div 2 = 0.5\)
90 years: \(0.5 \div 2 = 0.25\)
120 years: \(0.25 \div 2 = 0.125\)
So after 120 years, only 0.125 g would remain.