If the half-life of uranium-232 is 70 years, how many half-lives will it take for 10 g of it to be reduced to 1.25 g?
It will take 210 years.
The formula for radioactive decay is
#color(blue)(bar(ul(|color(white)(a/a) N/N_0 = (1/2)^n color(white)(a/a)|)))" "#
In your problem,
So, the uranium has decayed for 3 half-lives.