Partial means

The sum of all $64$ numbers is their mean times the number of numbers, which is $$64\times 64=4096$$

Similarly the sum of the first $36$ numbers is $$36\times 36=1296$$

Therefore the sum of the last $28$ numbers is $$4096-1296=2800$$

Therefore the mean of the last $28$ numbers is $$\frac{2800}{28}=100$$