Translation:It is 100 yen.
Wikipedia has an explanation of the history here. It's a bit complicated.
Wikipedia has an explanation of the history. It's a bit complicated. https://en.wikipedia.org/wiki/Japanese_yen#Pronunciation_and_etymology
Shouldn't it be ¥100? 円 goes after the number since it's a kanji and only really used in Japanese, but most (not all of course) currency symbols go before the number in English (some other languages, like French, put the € after the number while it goes before in English, for example). If you still get it wrong like that then report it.
This page might help explain, but the short version is that yen is what we call it, but since Japanese doesn't have a 'ye' sound anymore, the Japanese word is 'en' https://en.wikipedia.org/wiki/Japanese_yen#Pronunciation_and_etymology