21, Let AB be the lamp-post, the
lamp being at the position B and let MN be the man at
a particular time t and let AM = l metres Then, MS is
the shadow of the man Let MS = s metres Note that
∆MSN ~ ∆ASB
or
MS
AS  = MN
AB
or
AS = 3s (as MN =
2 and AB = 6 (given))
Thus
AM = 3s – s = 2s