what’s the mean 与你何干

与你何干： What does it have to do with you?

Regarding the reply, it depends on the thing you are discussing..

 1. What does it have to do with you.

 2. None of your business.

how to answer it :

1. A little free and easy:The dog bit Lu Dong bin…

2. Don’t suffer a loss: you are not saved!Or, you’re hopeless!

3. Interesting: I was wrong, you have no medicine to save. 

 4. Touching: I thought we were friends.

Expressing that you are curious about why s/he (want to) did certain things.

If your feelings aren’t in any way equivocal, you could simply say, 关你屁事。But apparently it’s offensive.