What is the sum of 15 and 15?"
In this example, the user would need to provide the correct answer (30) to pass the captcha verification successfully.
Keep in mind that captchas should be designed to be straightforward and easy for humans to solve while still being difficult for bots to bypass. You can also use different types of challenges, such as solving a simple math problem, identifying objects in an image, or typing characters from a distorted image.