A sequence that each term is related with the prior by a sum of a constant ratio is called a arithmetic progression, the sequence in this problem is one of those. In order to calculate the nth term of a setence like that we need to use the following formula:
an = a1 + (n-1)*r
Where an is the nth term, a1 is the first term, n is the position of the term in the sequence and r is the ratio between the numbers. In this case:
Let the average cost of car in 2009 be $x, then the average cost of a car in 2013 is 2x - 5 25,000 = 2x - 5 2x = 25,000 + 5 = 25,005 x = 25,005/2 = $12,502.50
