For this question you need to remember the standard deviation curve, commonly known as a bell curve. Here's a link to one for reference: http://statsdata.files.wordpress.com/2012/08/standard-d.gif.
If we have 30,000 miles as the average mileage (the center or mean as its shown in the picture) between 1,000 cars with the deviation being 5,000 miles (each s.d. on the picture), then our bell curve will show that approximately 34.1% of the 1,000 cars fall between 30,000 and 35,000 which is 341 cars (34.1% of 1000). We do the same going the other way, so between 25,000 and 30,000 miles there are 34.1% of the total 1,000 cars in that deviation (again, 341 cars). So to find the total amount of cars between 25,000 and 35,000, we add 341+341 and get our answer to be 682 cars.
The trickiest part is labeling the curve but after that it's all about taking the certain percent of the total amount of your sample.
I hope this helped. Let me know if you need it explained a bit more.