Asubstance has a half-life of 60,000 years and an initial mass of 1,000 grams. how many years will pass before only 250 grams of the substance's parent material is left?

30,000
90,000
120,000
240,000

After 120,000 years, only 250 grams of the substance's parent material will be left.

Explanation:

Half life is the time required for any isotope to decrease to its half of original mass or value during process of radioactivity. As half lie of substance is 60,000, by the definition, after 60,000 years the mass of substance will be decreased to (1000/2)= 500 grams i.e half of its mass.Similarly after next 60,000 years the mass of substance left will be half i.e (500/2)= 250 grams.Hence the total time for the substance to have only 250 grams left is (60,000 + 60,000)= 120,000 years.