(으)ㄴ데, 뭘 [Intermediate Grammar]

- (으)ㄴ데, 뭘

'-(으)ㄴ데, 뭘 그래요?' is abbreviated to -(으)ㄴ데, 뭘' or '-(으)ㄴ데, 뭐.' Usually, it is used when the speaker slightly raises an objection about the other party's action or an opinion or suggests an alternative. -기는요, 뭐' '지요, 뭐' are variant forms.

Example: 

가: 이 옷이 저에게 잘 안 어울리는 것 같아요. I don't think this outfit looks good on me.
 나: 아니요, 젊어 보이는데요, 뭘. No, you look young, don't worry.

가: 한국말을 잘하시네요. You speak Korean very well.
나: 잘하기는요, 뭘. No, I don't, don't say that.

가: 야유회갈 때 도시락은 각자 준비해야 하나요?
When we go on a picnic, should we make our lunch by ourselves?

나: 제가 한꺼번에 준비하지요, 뭘.

 Let me prepare it for everyone, don't worry.