How many milligrams of a 24 mg sample of cesium-137 remain after 120 years?

Question

How many milligrams of a 24 mgmg sample of cesium-137 remain after 120 years?

Answer 1

If HL is 30yrs, then the 24mg sample halves 4x in 120yrs ... 12, 6, 3, 1.5mg :)

Answer 2

Radioactive decay is 1st order. We ca use

FR = 0.5ⁿ

FR = fraction remaining = ?

n = number of half lives that have elapsed

You did not provide the half life so I will use an internet value of 30.2 years.

n = 120 yrs / 30.2 yrs = 3.97 half lives

FR = 0.5^3.97

FR = 0.0638

mg remaining = 0.0638 x 24 mg = 1.53 mg