I do not know how to solve this exercie from homewor, if someone can help me to solve $(3)$ and $(4)$ please. Thank you!

Independently observe a horizontal angle $5$ times with non$-$equal weight, the observations are:

$l_1=65{18}^{\text{'}}{42}^{\text{'}\text{'}},l_2=65{18}^{\text{'}}{50}^{\text{'}\text{'}},l_3=65{18}^{\text{'}}{40}^{\text{'}\text{'}}$

$l_4=65{18}^{\text{'}}{48}^{\text{'}\text{'}},l_5=65{18}^{\text{'}}{46}^{\text{'}\text{'}},$

The ratio between weight is $1$:$2$:$3$:$2$:$1.$ Please derive

$(1)$ The MPV of the horizontal angle.

$(2)$ Residual $V$ of every observation.

$(3)$ The unit weight standard deviation.

$(4)$ The standard deviation of each observation.

$(5)$ The standard deviation of the MPV $({}_x).$