​fourty electric bulbs are connected in series across a 220V supply.After one bulb is fused the remaining 39 are connected again in series across same supply.The %with which the illumination of the bulbs will change will be:

Percentage change in the illumination will be relatve change in the voltage drop across one resistance of bulb (before when bulbs are 40 and after when no of bulbs are 39).
Lets proceed to calculate the voltage drop across one bulb would be V₁ = I₁ R (since, I₁ = 220 / 40 R_bulb ….when bulbs were 40 of same resistance)
Similarly calculate the voltage drop across one resistance of 39 bulb series of same resistance (since, I₂ = 220 / 39 R_bulb).
Where the voltage drop will be more, that bulb system will be more brighter.

So drop in 40 bulbs would be 40 X I₁ R ….….….…..(a)
sim, drop in 39 bulbs would be 39 X I₂ R ….…. ..…. (b)

In this way calculate the change in illumination.....
Good Luck...............................