The sum of all $64$ numbers is their mean times $64=64^2$. Similarly the sum of the first $36$ numbers is $36^2$. Therefore the sum of the last $28$ numbers is $64^2-36^2=(64+36)(64-36)=100\times 28 = 2800$. Therefore the mean of the last $28$ numbers is $\frac{2800}{28}=100$.

*This problem is taken from the UKMT Mathematical Challenges.*

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