Number Theory

Since it's obvious there's more 2's than 5's in this string of numbers, we just count the number of 5's to get the answer/

5's: 10,15,20,25(25has 5^2), so there is 5^5 in total

from that we know there's 100000 that can be combined in this number which leaves us with 5 0's at the end

If I expand
$25*24*23* \dotsm *12*11*10$,
how many zeros are there at the end of the number I get?

$25*24*23* \dotsm *12*11*10 = 25!-9!$

$\begin{array}{|rcll|} \hline \text{zeros} &=& \lfloor\frac{25}{5}\rfloor +\lfloor\frac{25}{25}\rfloor -\lfloor\frac{9}{5}\rfloor \\ &=& 5+1-1 \\ \mathbf{\text{zeros}} &=& \mathbf{5} \\ \hline \end{array}$