# Head-Tail Coin Tosses

**Discrete Mathematics**Level 5

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.