Probability — Chebyshev bound for the sample mean of exponentials

Let $x_1, x_2, …, x_{10}$ be a sample of size 10 from an exponential distribution with mean $\theta$. The lower bound to the probability $P(0<\bar{X}<2\theta)$, where $\bar{X}=\frac{1}{10}\sum_{i=1}^{10}X_i$, is