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**risteek** This one is half-life I know but I cannot figure out how to go through it, it isn't finding the amount so none of the examples my teacher gave me can show how to move things around...any help is appreciated.

The half-life of sulfur-138 is 4 days. After 12 days,a sample of this radioactive material has been reduced to a mass of 0.64g.

a) What is the initial mass?

b) What is the mass after 2 days?

Also if it wouldn't hurt could someone could walk through it, if not then just show all the steps to solve it.