These videos may help:
GTA 5 - How to Make $2.1 Billion
GTA 5 - All Endings / Final Missions
GTA 5 - Michael's Best GIRLFRIEND Mission (Michael and Bride)
GTA 5 - Mission #75 - The Big Score (Subtle Approach) [100% Gold Medal Walkthrough]
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So, if we de some maths, the final mission count is going to be 276!