The price of the iphone can be modeled by the following expression:

where,
A: price of the iphone after t years
P: initial price = 1099
r: rate of percetage decrease in decimal for = 0.25
t: years
Then, the function becomes:

The price of the iphone after t = 3 years, according to the previous expression is:

Hence, the price of the iphone after 3 years would be $463.64