Grand Theft Auto: San Andreas/Missions/Triathlon
There are two triathlon competitions in GTA: San Andreas. They are both named "Beat The Cock" and are available on saturdays and sundays at Santa Maria Beach and Palomino Beach. Enter the challenge by going into the red marker that appears near the water when the challenge is available. Both of the triathlons are divided into three legs, beginning with the swimming leg, followed by cycling and then a running leg. The rewards for winning the triathlons are respectively $10000 and $20000.
The easiest "setup" for this triathlon, is to have low fat and low muscle, but high stamina, if not maxed. This should let you "sprint" through the swimming section, and should give you enough of a lead to win.
This is not required for 100% completion.