Number of Quadrilateral made by Bart in the classroom = 20
R =Number of quadrilateral having four right angles = {Square , Rectangle}=2
E=Number of quadrilateral having four equal side lengths = { Rhombus, Square}=2
Total favorable outcome =4 if quadrilateral with special properties are selected
Or
Total favorable outcome = 20 , if all quadrilateral is selected.
Out of Randomly selected quadrilateral which has four right angles, the quadrilateral which has four equal side lengths = R ∩ E ={Square}=1
Required probability=
or
.
Which is 5% if Total number of Quadrilateral = 20
And , 25% if total number of Quadrilateral selected = 4