Acar dealer is comparing his sales over the past two years. in 2006 he sold 175 cars in 2007 he sold 196 cars work out the percentage increase in the number of cars sold

You would subtract the smaller amount from the larger amount.
196 - 175 = 21
21 is how many more cars he sold in 2007 than 2006. So you'll now be able to put the difference and the smaller amount into a fraction
21/175
If you simplify it (divide 21 by 175) you get 0.12.
Since you need a precent, you'll times they by 100. Since 100%.
0.12 * 100 = 12.
You changed by 12%.