ABCD is a rhombus where angle B= 120 and BD=5 cm (Diagonal). Find out the perimeter of ABCD.

given: ABCD is a rhombus.
$\angle B=120 deg$ and BD = 5 cm
since diagonals of rhombus bisects the $angles. \angle ABD=\angle DBC=\frac{\angle ABC}{2}$
$$\begin{gathered} \angle ABD=\frac{120}{2}=60 \\ \angle BAD=180-\angle ABC=180-120 \\ \angle BAD=60 \end{gathered}$$
therefore in the triangle ABD;
$\angle ABD=\angle BAD=60$
therefore ABD is an equilateral triangle.
AB=AD = BD
AB = AD = 5cm
​perimeter of the rhombus ABCD = 4*AB
=4*5 
= 20 cm

hope this helps you