Answer
The amount of decay in the fourth year is $6.25 lbs$.
The total amount of decay in the first four year is $93.75 lbs$.
Work Step by Step
For the general term of the amount of decay is given by $a_{n} = 100(0.5)^n$ where n is the number of the year,
The amount of decay in the fourth year is $a_{4} = 100(0.5)^4 = 6.25 lbs$.
The total amount of decay in the first four year = $\sum\limits_{n=1}^4 a_{n} = \sum\limits_{n=1}^4 100(0.5)^n = 50 + 25 + 12.5 + 6.25 = 93.75 lbs$
