Solution: In your question, time is not given. So let's find the length of shadow after t second.
Let the boy of height 90cm be at point D on the ground from the lamp post after t sec.
Let time t=4 sec.
∴ AD = 1.2 m/sec × 4 sec = 4.8 m = 480 cm
Suppose the length of the shadow of the boy be x cm when he at position D.
∴ BD = x cm
In ∆BDE and ∆BAC
∠BDE = ∠BAC (90°)
∠DBE = ∠ABC (Common)
∴ ∆BDE ∼ ∆BAC (AA similarity)
BE / BC = DE / AC = BD / AB ( Corresponding sides are proportional )
90 / 360 = x / (480 + x)
1 / 4 = x / (480 + x)
4x = 480 + x
x = 480 / 3 = 160
∴ Length of boy shadow after 4 second is 160 cm.