Hello, jeunesse27!
Who wrote this problem?
The term "diameter" is usually reserved for circles.
A lamp with a parabolic reflector is shown in the figure. . Right!
The bulb is placed at the focus and the focal diameter is 20 cm. . latus rectum?
Find the width of the opening 15 cm from the vertex, to the nearest cm.
. . I'll take a guess at what's going on . . . Code:

15
C o     +     o D

F
(10,p)o    o    o (10,p)

* p *
*  *
*  *
    *o*         
V
The equation is: .
. . where is the distance from the vertex to the focus
The length of the latus rectum is
. . Hence: .
The equation is: .
If , we have: .
Therefore: .