#### Answer

$\mu=20$
$\sigma=20$
$P(\mu \leq t \leq \mu + \sigma) \approx 0.233$

#### Work Step by Step

We are given $f(x)=0.05e^{-0.05t}; [0;\infty]$
The mean of the distribution:
$\mu=\frac{1}{0.05}=20$
The standard deviation of X is
$\sigma=\frac{1}{0.05}=20$
The probability of the random variable between 1 standard deviation above the mean:
$P(\mu \leq t \leq \mu + \sigma) = P(20 \leq t \leq 40) \approx 0.233$