The Manager of a Fast-Food Restaurant Determines That the Average $3$

The manager of a fast-food restaurant determines that the average time that her customers wait for service is 2 minutes.
The manager wants to advertise that anybody who isn't served within a certain number of minutes gets a free hamburger. But she doesn't want to give away free hamburgers to more than $3$ % of her customers. What value of x must she use in the advertisement "if you aren't served within x minutes, you get a free hamburger"?