You bought airplane tickets for you and your wife to go to Germany on vacation. You bought them because the tickets were really cheap. Now your wife is angry because you didn't ask her first. You want to explain why you bought the tickets. You say:
It was a great deal. It seemed like a no-brainer.
"Getting a good deal on something" means buying something for a really low price. For example, if you're looking to buy a new car, you can say:
I haven't bought one yet because I want to wait around and get a really good deal on it.
You don't always have to use "get" and "on (something). You can just talk about "a good deal":
Five for ten dollars? That's a great deal!
Use this expression when you're explaining the choices that you made in the past. "It seemed like..." tells what you thought at that point:
It seemed like a good idea at the time.
It seemed like the car in front of me was going to stop, so I slammed on my brakes.
I didn't ask Joe about the money. It seemed like he was angry about something, so I didn't bring it up.
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.
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