Probability

Noting that probability is relative, and referencing the primitive crate I'm trying to mimic, everything in the primitive crate has an equal 5% chance of spawning (aside from scrap). I've not noted any circumstance where more than three items show up in the crate (one of which is scrap, which is always present). 

With 3 scrap always being present, then a chance for 1 or 2 more items being present, with each of the items having a 5% chance of showing up...

How would I replicate this? 

Also, once I'm finished with these crates, would you like a copy of the different .json files to share on here? I plan on eventually replicating most of the default crate loot for different tier of bots on my server.