Hello, would you please help me with this question for my final today?

4. Half-life is the amount of time required for the mass of a radioactive isotope to decay to half of its initial amount. The half-life of the isotope cesium-137 is 30 years. Suppose we have a 10 gram sample.

(a) Find an exponential equation that models the mass remaining after t years.

(b) How much of the sample will remain after 80 years?

(c) After how long will only 1 2 grams of the sample remain?