Hey there!
A half-life means after a certain amount of time, half of that substance will be gone/changed after that time.
How many half-lives are in 16 days?
Multiply 16 by 24 (to get the total hours) and divide that by 2.5.
That is 153.6 half-lives.
35(0.5)^(153.6) represents the equation for this.
Simplify this and get 8.88 x 10^(-45).
There will be 2.02 x 10^(-45)g of germanium-66 sample remaining after 16 days.
Hope this helps!