Takuya's parents gave him $100dollar as a birthday gift. Since he loves board games, he spent $20dollar out of his present on board games at the end of each month until his money ran out. Let f(n) be the amount of money that remained from Takuya's present during month n, where n=1 represents the month he received his present.
f is a sequence. What kind of sequence is it?
p. s" tell me if it is a geometric sequence or arithmetic and find the explicit formula