Answer:
Part 1 : 
Part 2 : Half life is 84 minutes ( approx )
Step-by-step explanation:
Part 1 : Suppose the function that shows the amount( in grams ) of the substance after t minutes,

If t = 0 min, A = 250 grams,


If t = 250, A = 32 grams,



Taking ln both sides,


Hence, the equation that shows this situation,

Part 2 : If A = 250/2 = 125,


Taking ln both sides,


Therefore, the half life of the substance would be 84 minutes.