You cant just add the percents. This is not how probability works. This is from the same ppl that brought us 1000*40% is 1040 and couldnt put the right value into an event % xp bonus and showcased a bugged ball lightning build in pit with number values wrong.
IF you could just add the percents then say it was 5 chances to drop a mythic at 20%. According to Bliz you would have 100% chance to have a mythic after 5 drops. But obviously there is a chance you would not have a mythic. Laughable maff skills.