Answer
$\text{Mean}=17$
$\text{Median}=18$
$\text{Mode} = 13$
Work Step by Step
$\underline{\text{Mean}}:$
$\text{Mean}$ = $\dfrac{\text{Sum of all observations}}{\text{Total no. of observations}}$ = $\dfrac{13+21+18+13+20}{5}=\dfrac{85}{5}=\boxed{17}$
$\underline{\text{Median}}:$
$\text{We can find the median by first arranging the given observation in the increasing order and find the mid-point of these observations. In this case, we get it as}$
$13, 13, 18, 20, 21$
$\text{There are 5 observations. So, the mid-point of these observations is given as}$
$\text{Mid-point} =\dfrac{n+1}{2}=\dfrac{5+1}{2}=\dfrac{6}{2}=3$.
$\text{We have found out that the $3^{\text{rd}}$ observation of the arranged number is the median. The $3^{\text{rd}}$ observation in this arranged numbers is 18.. That means, the median of this problem is $\boxed{18}$}$.
$\underline{\text{Mode}}:$
$\text{Mode is the number that occurs the most. In this case, the number that occurs the most is $13$. So, the mode of this question is $\boxed{13}$}$
