Is this exponential decay and how do i even solve this? Please help

Question

A radioactive element decays exponentially. Initially, there are 100 grams. 8 days later, the element has decayed to 60 grams. At what time will the element have decayed to 25 grams?

Answer 1

Radioactive decays are first order reactions

ln(At/Ao) = -kt

Ao = 100g, t = 8 days, At = 60 g , Substitute and find k value

Now, with this value of k use the equation again, this time to find t

Ao = 100g, At = 25g, k we found, substitute and find t