A) 60 W
B) 180 W
C) 10 W
D) 20 W
Correct Answer: D
Solution :
Let \[{{R}_{1}},{{R}_{2}}\]and \[{{R}_{3}}\]are the resistances of three bulbs respectively. In series order,\[R={{R}_{1}}+{{R}_{2}}+{{R}_{3}}\] but\[R={{V}_{2}}/P\]and supply voltage in series order is the same as the rated voltage. \[\therefore \] \[\frac{{{V}^{2}}}{P}=\frac{{{V}^{2}}}{{{P}_{1}}}+\frac{{{V}^{2}}}{{{P}_{2}}}+\frac{{{V}^{2}}}{{{P}_{3}}}\] or \[\frac{1}{P}=\frac{1}{60}+\frac{1}{60}+\frac{1}{60}\] or \[P=\frac{60}{3}=20\,W\]
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