Hi Banach334,
To start, you'll likely get more of a response if you post this in the Quant section:
gmat-quantitative-section-7/Percent questions come from the broader category of 'ratios', meaning that you'll see several questions during the quant section that 'look different', but test you on the same general math concepts. In this category, you can approach the math in a variety of different ways, so you don't necessarily have to use (X/100) - using a simplified fraction or decimal-equivalent often makes the math far easier to complete.
Since you don't have a specific example to refer to, you might want to search the Quant Forums for questions that have been tagged as 'percent' questions. I'll be happy to talk you through any of those prompts if you'd like my 'take' on them.
GMAT assassins aren't born, they're made,
Rich