Question

Math question ,how ?

Answer 1

Let x be height of the shadow
30/15 = 90/x
Do cross multiplication
30x =1350
x = 45

Let h= height of the screen
1/4 of 45 is 11.25
H = 11.25 + 45
H= 56.25

There's your answer. I hope I did it right.

Answer 2

Height of the screen = x
height of the shadow = $\frac{3}{4}x$

ΔAEC ∼ ΔADB  ⇒

$\frac{BD}{AB}= \frac{CE}{AC} \\ \\ \frac{15}{30}= \frac{ \frac{3}{4}x }{90+30} \\ \\ \frac{1}{2}= \frac{ \frac{3}{4}x }{120} \\ \\ \frac{3}{4}x = \frac{120*1}{2}=60 \\ \\ x=60: \frac{3}{4}=60* \frac{4}{3}=20*4=80 \ cm$

Height of the screen is 80 cm.