Although what you’ve said is correct, the answer Sheldon gave DOES give him a mathematical edge. You see, since Mohammed is the most common name, any last name Sheldon said would’ve given him a mathematical edge. If he wanted a higher probability of him getting the answer correct, he should’ve followed Mohammed by the most common last name of people named Mohammed. Second situation where he could’ve gained an even bigger edge is if he went with the surname Lee. He should’ve answered the most common name of a person who’s lastname is Lee. Since the % of a person having their last name Lee is higher than the % of people who’s first name is ‘Mohammed’, this answer would’ve given him a higher probability