(8 pts) 7. (a) A radioactive substance has a half-life of 120 years. Determine an equation that gives the amount left of the sunstance if the initial amount (when time / = 0) was 2000 grams. (b) Complete the following table of values of the amount left for the given choices of the time f (years). I (years) Amount Left 0 2000 120 240 500 1000 (c) How long will it take for the amount of the substance left to be 10 grams