When a vocab word is a single kanji with no okurigana, it usually uses the kun'yomi reading. Numbers are an exception to this. Numbers use the on'yomi reading, which you've already learned by learning the kanji's reading! How nice!
I read one hundred pages in one week.
When you get to Level 100, Prince, you can change your job from prince to king or queen.
I can buy a bunch of different things with a hundred yen.