A small coin nano-challenge

[brickviking]

I saw Valentin's #minichallenge, and my eyes glazed over after I saw the first question. So I thought I'd come up with my own nano-challenge after an event that actually happened to me.

Caveat: I haven't managed to "prove" this one myself, it seems I left school with passable maths that have degraded since for lack of use. It's also dependent upon whose currency you choose to implement this in.

I have a single coin at home. I went out one day, and found another coin. "Ooo goodie", I thought, "I've got x money now." When I got home, I found I had only ended up with half the money I thought I had.

What two coins would make this true, and for whose currencies is it currently valid? In my case, only one set of coins meets the stated properties. I'm also talking only about coins in current circulation, not commemorative coins.

(Post 328)