If the number increases each year, then the 15% starts with a bigger number each year.
The first increase raises 300 million to (1.15 times 300 million) = 345 million.
The next increase raises 345 million to (1.15 times 345 million) = 396.75 million.
and so on and so on.
This is just like compound interest in a bank. Each time the bank
pays you interest on your savings, it pays interest on a bigger amount.
In this problem, the number of cars increases
... at the end of 2000 / beginning of 2001
... at the end of 2001 / beginning of 2002
... at the end of 2002 / beginning of 2003
... at the end of 2003 / beginning of 2004 .
That's four times. Each increase raises it 15% higher than it was before.
So you need to find
(1.15) · (1.15) · (1.15) · (1.15) of 300 million.
The way to write that is
(300 million) · (1.15)⁴ = 524 million 701 thousand 875 cars.
Rounded to the nearest whole million, that's 525 million.