Hi, there. I happy to help with this.

Sarah paid a sales tax of 8% on her purchase. If the sales tax had only been 5%, she would have paid $12 less on her purchase.

The difference of those two percents is 8 - 5 = 3%, so this is saying 3% of her purchase is $12. If 3% is $12, then 1% is $4, and 100% is $400.

That's a method you can always use when given two different percent values and the difference between them.

For more general strategies on percent, we have a whole set of video lessons at Magoosh that explain everything you'll need to know about percents for the GMAT.

Here's a sample lesson:
https://gmat.magoosh.com/lessons/209-wor ... h-percents

Here's another percent problem you can practice.
https://gmat.magoosh.com/questions/30
After you submit your answer to that question, the next page will have a full video explanation. Each one of our 800+ of our practice questions has its own video explanation.

Let me know if you have any questions.

Mike