How long will it take for 60 grams of Californium-251 to decay to leave a total of 7.5 grams? Californium-251 has a half-life of 800 years

Answer 1

60*(1/2)^(t/800) = 7.5
t = 2400 years

makes sense, since 7.5/60 = 1/8, or 3 half-lives