An obvious conclusion or an easy decision is a no-brainer. Adding 2 + 2 is a no-brainer, and so is the decision to accept a generous scholarship.
The informal no-brainer describes choices that are so clear they're hardly even choices: they require almost no thought, since one alternative is obviously better. Saying yes to the the prize of a trip for four to Disney World is a no-brainer, and when you're shopping for a new laptop, buying the one with more memory that's less expensive is a no-brainer. No-brainer stems from the idea that it doesn't take brains to make a decision so simple.