Question

A radioactive substance has a half-life of 1.25 million years. What is the age of a rock in which 6.25% of the original radioactive atoms remain? 5 million years old 6.25 million years old 3.75 million years old6 2.5 million years old 1.25 million years old 625 thousand years old

Answer 1

Answer:

The age of the rock is 5 million years

Explanation:

Fraction of radioactive substance remaining = 6.25% = 625/10000 = 1/16

Number of half-lives undergone by the substance to leave behind 1/16 of the original sample

1/16 = (1/2) * (1/2) * (1/2) *(1/2)

Therefore, the substance have decayed 4 times

One half=life = 1.25 million years

Four half-lives = 4 * 1.25 million years = 5 million years

Therefore, the age of the rock is 5 million years