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Having trouble getting all 70 sponsor challenges complete? I have the solution. The sponsor challenges end at 40 because the challenges recognize the fact that you have full stats once you do 40, and they will stop doing so when your stats are full. To beat all sponsor challenges, what you do is, before you accept the challenges, set all of your stats to 3, then take the challenges. It's harder, but it lets you do all of them. Trust me, this worked for me, so it should work for you.
P.S. *WARNING: SPOILER ALERT* Unfortunately, all you get from doing every sponsor challenge is bragging rights. You don't unlock anything.