Math Half life equations

Question

The half-life of Palladium-100 is 4 days. After 12 days a sample of Palladium-100 has been reduced to a mass of 4 mg. What was the initial mass (in mg) of the sample? 

What is the mass 6 weeks after the start? 

Answer 1

4 = a₀(0.5)^12/4, calculate

a(t) = a₀ (0.5)^28/4, use the value of a₀ from previous