Half life problem

Question

The half-life of Palladium-100 is 4 days. After 20 days a sample of Palladium-100 has been reduced to a mass of 3 mg.

What was the initial mass (in mg) of the sample?

What is the mass (in mg) 7 weeks after the start?

Answer 1

3 = a₀(0.5)^(20/4)

3 = a₀(0.5)⁵

3 = a₀(0.03125)

96 = a₀

A(28) = 96(0.5)^(28/4)

Answer 2