### Solution

We are looking for how many more cars that were electric in $2005.$ Let's call this number $x.$ In $2004$ there were $5$ million electric cars which increased by $x$ to $8$ million vehicles in $2005.$
$5000000+x=8000000 $
By isolating $x$ we can find how many more cars that were electric in $2005.$
There were $3000000$ more electric cars in $2005.$

### Hint

### Answer

$3000000$