The half-life of strontium-90 is 25 years. How much strontium-90 will remain 100 years if the initial amount is 4.0 g

Ok so because the half life is 25 years, the mass will "decrease" by half every 25 years.

So after 25 years it becomes 2 grams,

At 50 yrs it becomes one gram

At 75 years it will be half a gram

At 100 years it will be a quarter of a gram or 0.25 grams.

I. e. the final mass is 0.25 grams