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.