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**ANDS!** Assuming you are drawing a number with replacement, each person choosing a number is a single independent experiment where the odds of success are 1/100, and the odds of failure are 99/100. So your distribution would be:

$\displaystyle X~Bin(100,0.01)=$

$\displaystyle P(X=x)=\left(\begin{array}{c}100\\x\end{array}\rig ht)\left(\frac{1}{100}\right)^{x}\left(\frac{99}{1 00}\right)^{100-x}$

And you are trying to find P(X=3). It's really small. As it should be. For your second part, you are finding $\displaystyle P(X\geq3)$. This would be insane to do for x=3, 4, 5. . .100 - so simply calculate the compliment and subtract it from 1:

$\displaystyle 1-P(X<3)$;

Here you are only solving three, so it shouldn't be that bad.