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Hello I am having trouble with a slightly biological problem

I am given a regular expression $- [RK]-[LV]-[DE]-x(2)-Y$

this expression means that there is a string with the first position being an $R$ or a $K$, the second position an $L$ or a $V$, third a $D$ or an $E$, fifth and sixth can be anything, and the last position must hold a $Y$.

These characters represent amino acids with which there are twenty. (odds of first position being a match is $2/20 - 1/20+1/20$).

I am having trouble calculating the variance of the set assuming positional dependency.

thank you very much for your help and I will clarify anything if necessary.

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