Strontium-90 has half-life of 28 years. if a 1.00-mg sample was stored for 112 years. what mass of Sr-90 would remain?

Answer 1

k = 0.693/t_1/2

Substitute k in the following:
ln(No/N) = kt
No = 1 mg
N = mg remaining after t time
k from above.
t = 112 years.