Cafe F18 will throw mysterious problems (customers) at you, one problem per 30 seconds. Figuring out how to solve these problems is up to you, though at the start guessing might be helpful.

Your goal is to get the time at the top of the screen to zero. This time will decrease faster the fewer unanswered problems (unserved customers) you have. On the other hand, if you leave a problem unanswered until its time left reaches 0, 3 / 4 of its time left is added to the time at the top of the screen (but it does stop counting as an unanswered problem for the purposes of decrease speed). If you incorrectly answer a problem, the problem remains and 1 / 4 of its time left is added to the time at the top of the screen. When you answer a problem correctly, it goes away (your time at the top of the screen doesn't immediately change, but it starts decreasing faster because you have fewer unanswered problems).