Would this be rite?
A(lamp)

 ...................... C (anita)
12.................... 
 ...................... 5
 .......................
B ....... x ......... D ........................................ H (Tip of head)
Join RAY> ACH
anita moved distance x in time (t)
x = speed * time = 4t
> let DH = y = length of shadow
> angle AHB = CHD = p
tan p = 12/(4t+y)
tan p = 5/y
5/y = 12/(4t+y)
5(4t+y) = 12 y
7y = 20 t
y = length of shadow = 20t/7 >>>>>>>>>>>>>>
...
D = BH = x + y
D = 4t + 20t/7
D = [28t + 20t]/7
D = 48 t/7