Which is greater: 18% of 250, or 25% of 180?

18% of $250=\frac{18}{100}\times 250=45$. 25% of $180=\frac{25}{100}\times 180=45$. They are equal. This works because $\frac{18\times 250}{100}$ and $\frac{25\times 180}{100}$ contain the same product, 4500. The trap is judging by eye — 25 is the bigger percentage, so 25% of 180 "looks" bigger, but a percentage means nothing until you know the quantity it is taken of.