9 fair coins are independently tossed in a row. Let $X$ be the random variable denoting the number of instances in which a Head is immediately followed by a Tail during these 9 tosses. The variance of $X$ has the form $\frac {a}{b}$, where $a$ and $b$ are coprime integers. What is the value of $a+b$?

**Details and assumptions**

Note: If we have 5 coins which are tossed as $\underline{HT}T\underline{HT}$, there are 2 instances in which a Head is immediately followed by a Tail; they have been underlined.

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