The reading of this word is the kun'yomi reading of the kanji 五, meaning you didn't learn it when you learned the parent kanji reading. To remember this word, think of it this way:
You have five things. What do you have to do with them? You need to eat (いつ) them. If you come up with something crazy you have to eat, and focus on the fact that you must eat five of these things, it will help a lot too.
I ate five pieces of mochi.
Fifty white female Americans are competing against each other to obtain five manju.
There are five hardships in life and one of them is migraines.