Chances

Since everything works on chance,
a chart showing how the chances increase with multiple draws is useful

This chart was created to show how the chances increase
if you buy more chests with a 20% chance of getting a legendary item.

The funny part of the whole chart is the fact that nowhere is there a 100% chance.

You can just be lucky and get the item you want on the first try when the chance is 1% (top left) or be so unlucky that you don't get the item you want even on the 100th try when the chance is 99.99999999999999999999999999999999999999999999999999999999999999999997472% (down right).
Am not kidding- That's actually that number.
The calculation is simple. (1 - 0.20^{100})