“For example, if the half-life of a substance if 5 years, if we begin with 200g, how much substance will there be in 20 years?” Jamie asked.

“After 5 years there will be 100g, after 10 years there will be 50 grams, after 15 years there will be 25 grams, and after 20 years there will be 12.5 grams,” Mandy answered.

“Another example,” Jamie continued, “if the initial quantity is 100g, the current quantity is 25g, and the time taken is 10 years, what is the half-life?”

“Going from 100 to 50 to 25, there have been 2 half-lives,” Mandy thought, “This means each half life has taken 5 years .”