A "no-brainer" is a decision that's really easy to make because it's very obvious what the best choice is. For example, if someone offers you a job where you can earn twice as much as you do now, taking the job is "a no-brainer":

When Chuck made the offer, it was a no-brainer. I immediately accepted.

In the example at top, the husband thought that buying the tickets was a no-brainer because of the low cost. But his wife disagreed.

You can remember "no-brainer" by imagining a decision that's so easy to make, even a person without a brain can make the right choice.