The half-life of strontium-90 is 25 years. How much strontium-90 will remain after 100 years if the initial amount is 4.0 g?
If the half-life of Sr-90 is 25 years it means that after 25 years its amount will be half the initial quantity.
If you have 4,00 g, after 25 years you have 2,00 g;
after 50 years you have 1,00 g;
after 75 years you have 0,50 g;
after 100 years you have 0,25 g