The line drawn through the midpoint of one side of a triangle parallel to another side the third side.

In the figure given below, $S$ and $T$ are the midpoints of $PQ$ and $PR$ respectively. If $ST=3 \mathrm{cm}$ then $QR=$ _____ $\mathrm{cm}$.

In the figure given below, $\mathrm{D}$ and $\mathrm{E}$ are the midpoints of $\mathrm{AB}$ and $\mathrm{AC}$ respectively. If $\mathrm{DE}=7.5 \mathrm{cm}$, then $\mathrm{BC}=$_____ $\mathrm{cm}$.

Find the value of $x$ in the given triangle.

Find the value of $x$ in the given triangle.

Find the value of $x$ in the given triangle.

In $\Delta ABC$, $D$ is the midpoint of $AB$, $E$ is the midpoint of $AC$. If $DE=l \mathrm{cm}$ when $BC=6 \mathrm{cm}$, find the value of $l$.

In $\Delta ABC$, $D$ is the midpoint of $AB, E$ is the midpoint of $AC$. Calculate $\angle ADE$, if $\angle DBC=140°$. Express your answer in degree.