Taken from MeonOne's guide for the 360 version of the game.
To reach 100% completion you will have to complete the following:
*Glitch Warning: If you have completed all the missions, cases, favors, jobs, races and events and you are stuck a certain percent. Just go and redo the first Martial Arts Club, the one located next to the Kung Fu temple where you learn your fighting moves.
• 30 Traid Missions
Show up as Green badges on your map.
• 4 Cases
Show up as Dark Blue badges on your map. Cases are also divided into series of missions before completing one case.
• 36 Favors
Show up as Yellow circles with a black person in the middle. All the favors will unlock through natural progression.
• 30 Jobs
These include the 20 Tran jobs, Roland Ho 5 jobs, the 4 Martial Arts Club and 1 hijacking of a security van.
• 18 Races
These appear on the map as Orange race Flags. 2 out of the 18 races unlock when you take Sandra out on a date. For some of the races you will need to have a class A, B, C vehicles to be able to compete in it.
• 28 Events
Events include:
17 Drug Bust (Light Blue)
4 Shoplifters (Yellow)
3 Hit and Runs (Red)
4 Junk in the Trunks (Orange)
Here is a map of all open world Events.
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