We know that,
P = 60 W
V = 250 V
Let us consider,
P = VI
I = 60 / 250
This implies I = 0.24 A
We know
Resistance of lamp R = V2 / P
R = (250)2 / 60
R = 1041.6 ohm
If the voltage drops to 200 V, the power consumed is
P = V2 / R
P = (200)2 / 1041.6
P = 38.4 W
Therefore, consumed decreases to 38.4 W