How to Calculate the Remaining Mass of a Radioactive Substance Over Time?

[mathdrama]

Basically need more help checking my answers, I think it's better if I provide the full context:

A radioactive substance with an initial mass of 300mg has a half-life of 2 years.

a) Write an equation to model the mass of the material over time.

Let A = mass of the material
Let t = time

A = 300*2^(-t/2)

b) What mass will remain after 12 years?A = 300*2^(-t/2)
A = 300*2^(-12/2)
A=300*2^(-6)
A=300*1/2^6
A=300*1/64
A=75*1/16
A=75/16
A=4.6875

 

[Opalg]

That answer is numerically correct, but you should state the units: A=4.6875mg.