Drop rates

Hey guys.

So i'm trying to create a boss with multiple drops (like 5 differents drops) and i wanted to know if it's possible to create a probability for each items. For exemple : 1% for a diamond block, 5% for a gold block, 20% for an iron ingot... for 5 differents items with a low probability for the boss to drops nothing. I tried many procedure but it always fail. Thanks for reading and helping me !