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

Ithink it’s 3 as the answer.