Arrangement of 5 letters with a specific order

Question

how to calculate the arrangement of the 5 letters a,b,c,d,e when a,b,c must be in the written order: abcde,adbce,daebc,...

I know the answer is 20 by another way but want to know what's wrong with my following method:

OOaOObOOcOO so we need to select 2 place of from the 8 'o's ,that's the combination c(8,2) and considering their order we have 2!*c(8,2) . I know that's wrong but don't know why!

Answer 1

It's wrong because the four choices

dOaOObOOcOe
dOaOObOOceO
OdaOObOOcOe
OdaOObOOceO

all give the same final order, but your method counts them as distinct.