The 20% RE does NOT work like this:
RE 1 item = 20% chance; therefore RE 5 items = 100% chance! I r so smart
Rather, it works like THIS:
Will RE X items:
Item 1 has 20% chance of success, 80% chance to fail
Item 2 has 20% chance of success, 80% chance to fail
Item 3 has 20% chance of success, 80% chance to fail
...
Rinse and repeat until you hit the 20% chance.
In short: The chances of success are rated individually, not cumulative.