Solve the Problem $\mathrm { a } _ { \mathrm { n } } = 100 ( 0.2 ) ^ { \mathrm { n } }$

Solve the problem.
-The amount of decay in kilograms of a radioactive isotope each month is given by the sequence whose general term is $\mathrm { a } _ { \mathrm { n } } = 100 ( 0.2 ) ^ { \mathrm { n } }$ , where $\mathrm { n }$ is the number of the month. Find the amount of decay in the third month, and find the total amount of decay in the first three months. Round to the nearest tenth, if necessary.

A) $0 \mathrm {~kg} ; 0.2 \mathrm {~kg}$
B) $60 \mathrm {~kg} ; 120 \mathrm {~kg}$
C) $800 \mathrm {~kg} ; 1400 \mathrm {~kg}$
D) $0.8 \mathrm {~kg} ; 24.8 \mathrm {~kg}$