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

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​If

(

)
s(t) represents the value, in dollars, of the stone t years after Grandma Gigi gave it to Leyla, determine the value of a and r if

(

)
=

(

)

s(t)=a(r)
t
.