Strontium-$90$ has a half-life of $28$ days. Answer the following questions.

---

1. A sample has a mass of $75$ mg initially. Find a formula for the mass remaining after $t$ days.
Answer: $m(t) =$

2. Find mass remaining after $60$ days.
Answer (in milligrams):

3. How long does it take the sample to decay to a mass of $1$ mg?
Answer (in days):

---