Answer:
R = 40 ohms
Step-by-step explanation:
Two bulbs have a resistance of 20 ohms when working normally.
When they are placed in series, the combined resistance is given by :
R = R₁ + R₂
We have, R₁ = R₂ = 20 ohms
R = 20 +20
= 40 ohms
Hence, the combined resistance of the two bulbs is 40 ohms.