Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?

Since two bulbs are in series they will get equal amount of  electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.