Question

14 g of a radioactive substance are present initially and 4 yr later only 7 g remain, how much of the substance will be present after 7 yr?

Answer 1

The annual ratio of decrease of the substance is

$\frac{7g}{4yr}$

After 7 years the substance will decrease:

$\frac{7\text{ g}}{4yr}\cdot7yr=\frac{49\text{ g}}{4}=12.25g$

The amount of substance after 7 years must be 1.75 g