If a sample of Sr-90 initially contains 1.1 mmol of Sr-90, how many millimoles are left after 25.20 years?
any ideas on where to start??
Yes.
See your post above.
First calculate k using the half life, then use
ln(No/N) = kt
Use No = 1.1 mmol
N = unknown
k from half life
t = 25.2