be a no-brainer
To be a very easy or obvious choice, decision, or solution. The military said they would pay off my student debt and provide a guaranteed job after I completed four years of service, so it was a no-brainer for me to enlist. The answer to the riddle ended up being a total no-brainer. I don't know why it took me so long to figure out! Wait, if it's an all-expenses-paid vacation, then I'm definitely in—that's a no-brainer!
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