Strontium-90 has a half-life of about 29 years. After 65 years, about how many grams of a 25-gram sample will remain?

Ending Amount = Beginning Amount / 2 ^ n

where 'n' = number of half-lives n = 65 / 29 = 2.2413793103

Ending Amount = 25 g / 2^2.2413793103

Ending Amount = 25 / 4.7284892286

Ending Amount = 5.2871009727 g = 5.29 g (rounded)