Example 1:

Bill: Do you have plans for vacation, Mike?
Mike: Nope. I can’t decide what I want to do. My roommates want to go to Las Vegas, but my parents want to take me on a cruise through Mexico.
Bill: Mike! That’s a no-brainer! You can go to Vegas anytime! The cruise to Mexico is something you shouldn’t pass up.

Example 2: 

Seth: Have you heard back from the company you interviewed with last week? I know you’ve been looking for work for a few months.
Ruth: Yes, actually. They called me the other day to offer me a position.
Seth: Wow! That’s great! You’re going to take the job, right?
Ruth: Of course! It’s a no-brainer!

Explanation:
A no -brainer is an extremely easy question or decision to make.
In Example 1, Mike has to decide between a trip to Las Vegas with his friends or a cruise to Mexico with his parents. While both trips sound fun, Bill thinks the choice is obvious and easy to decide- of course he should go to Mexico! Living in southern California, Mike has plenty of opportunities to travel to Las Vegas, but it’s unlikely his parents will take this same cruise again.
In Example 2, Ruth says she will take the job. To her, there is no other option. It is the obvious choice.
You can think of something as a no-brainer if someone who has no-brain can still make the decision easily.

https://languagesystems.edu/