Aileen plays badminton where she and her opponent stand on opposite sides of a net and attempt to bat a birdie back and forth over the net. A player wins a point if their opponent fails to bat the birdie over the net. When Aileen is the server (the first player to try to hit the birdie over the net), she wins a point with probability $\frac{9}{10}$ . Each time Aileen successfully bats the birdie over the net, her opponent, independent of all previous hits, returns the birdie with probability $\frac{3}{4}$ . Each time Aileen bats the birdie, independent of all previous hits, she returns the birdie with probability $\frac{m}{n}$ , where $m$ and $n$ are relatively prime positive integers. Find $m + n$.
1. We start by noting that Aileen wins a point if her opponent fails to return the birdie. The probability of Aileen winning a point when she is the server is given as $\frac{9}{10}$.

2. Let $p$ be the probability that Aileen successfully returns the birdie. We are given that her opponent returns the birdie with probability $\frac{3}{4}$.

3. The probability that Aileen wins the point on her first serve is $\frac{1}{4}$ (since her opponent fails to return the birdie).

4. If her opponent returns the birdie (with probability $\frac{3}{4}$), then the game continues, and we need to consider the probability of Aileen eventually winning the point. This can be modeled as an infinite geometric series.

5. The probability that Aileen wins the point after the first return by her opponent can be written as:
   \[
   \frac{m}{n} \cdot \frac{1}{4} + \left( \frac{m}{n} \cdot \frac{3}{4} \right) \cdot \frac{m}{n} \cdot \frac{1}{4} + \left( \frac{m}{n} \cdot \frac{3}{4} \right)^2 \cdot \frac{m}{n} \cdot \frac{1}{4} + \cdots
   \]

6. This series is a geometric series with the first term $a = \frac{m}{4n}$ and common ratio $r = \frac{3m}{4n}$.

7. The sum of an infinite geometric series is given by:
   \[
   S = \frac{a}{1 - r}
   \]
   Substituting the values, we get:
   \[
   \frac{9}{10} = \frac{\frac{m}{4n}}{1 - \frac{3m}{4n}}
   \]

8. Simplifying the equation:
   \[
   \frac{9}{10} = \frac{\frac{m}{4n}}{\frac{4n - 3m}{4n}} = \frac{m}{4n - 3m}
   \]

9. Cross-multiplying to solve for $m$ and $n$:
   \[
   9(4n - 3m) = 10m
   \]
   \[
   36n - 27m = 10m
   \]
   \[
   36n = 37m
   \]
   \[
   \frac{m}{n} = \frac{36}{37}
   \]

10. Since $m$ and $n$ are relatively prime, we have $m = 36$ and $n = 37$.

11. Therefore, $m + n = 36 + 37 = 73$.

The final answer is $\boxed{73}$.