To write this ratio in simplest form, we simply need to take all the common factors on both sides and divide.
For this one, we can see that both 8 and 20 are divisible by 2, so we will start with that.
8:20 = (8 / 2):(20 / 2) = 4:10
And these both still are divisible by 2 yet again, so we can continue dividing.
4:10 = (4 / 2):(10 / 2) = 2:5
And that's our answer.
8:20 in simplest form is 2:5.
Hope that helped! =)