Question 943702
Write out {{{20!}}} fully to get


20!= 20x19x18x17x16x15x14x13x12x11x10x9x8x7x6x5x4x3x2x1


Now I'm going to highlight in <font color="red">red</font> the factors that are multiples of 5


20!= <font color="red">20</font>x19x18x17x16x<font color="red">15</font>x14x13x12x11x<font color="red">10</font>x9x8x7x6x<font color="red">5</font>x4x3x2x1


So we have 20, 15, 10, 5


20 = 5*4
15 = 5*3
10 = 5*2
5 = 5*1


We have four copies of '5' being multiplied here. So we have {{{5^4}}} as a factor of {{{20!}}}