Grandma gigi gave leyla a rare purple stone for her sweet 16 birthday. at that time, the stone was worth$245. it has been increasing in value by the same percentage each year, when leyla turned 21, the stone was worth $560.50. if s(t) represents the value in dollars of the stone t years after grandma gigi gave it leyla determine the value of a and r if s(t)=a(r)