Quick HELP NEEDED!! (general maths) (1 Viewer)

[freakinout]

im doin question 25 a) ii) in the 2002 HSC paper can anyone help me.... id type it up but its a diagram... come on ppl its good practice...PLZ

 

[shortygb]

the 2 triangles are similar to each other.

so therefore
d
200

equals

20
x

solve it making d the subject like you would normally do
d = (20/x) x 200
d = 4000/x

or for x

x = 4000/d