How long will it take for a 28 g sample of 236Ra to decay to 3.5 g?

Answer 1

We need to know the half life of 226Ra, assuming that's what you meant, and not 236Ra, which I don't believe exists. Looking it up, I find it to be 11.4 days.

We can use the simple formula of

Fraction remaining = 0.5ⁿ where n = # of half lives that have elapsed.

Fraction remaining = 3.5 g/28 g = 0.125

0.125 = 0.5ⁿ

n = 3 half lives

3 half lives x 11.4 days/half life = 34.2 days