I'm pretty sure the chance % only applies to SUCCESSIVE attempts ie if there is a 20% chance of success and you try five times in one go, one of the attempts will likely be successful, as opposed to trying 3 times in one go, going back to replay missions to earn enough gems and then trying another three times. Always have…
Just install the games (gun Bros multiplayer, deer hunt reloaded, BT gangsta etc) and play every day to get the daily bonuses. The credits will be available to u across all games. Gun Bros gives u 48 credits after five consecutive days of play!