b
વાલ્ટેજ \(V\) આપતાં બન્ને ફિલામેન્ટ પાવર \(P_1\) અને \(P_2\) લે છે.

અહીં, ઉષ્મા \({\text{H  =  }}{{\text{P}}_{\text{1}}}{{\text{t}}_{\text{1}}}{\text{  =  }}{{\text{P}}_{\text{2}}}{{\text{t}}_{\text{2}}}{\text{  =  (}}{{\text{P}}_{\text{1}}}{\text{  +  }}{{\text{P}}_{\text{2}}}{\text{)t}}\,\) તથા  \({P_1}\, = \,\,\frac{H}{{{t_1}}}\,\,\) તથા \({P_2}\, = \,\,\frac{H}{{{t_2}}}\)

\(\therefore \,\,t\,\, = \,\,\frac{H}{{{P_1} + {P_2}}}\,\,\)

\( = \,\,\frac{H}{{\frac{H}{{{t_1}}} + \frac{H}{{{t_2}}}}}\)

\( = \,\,\frac{1}{{\frac{1}{{{t_l}}} + \frac{1}{{{t_2}}}}}\,\, = \,\,\frac{{{t_1} \times {t_2}}}{{{t_1} + {t_1}}}\)

\( = \,\,\frac{{10 \times 15}}{{10 + 15}}\,\, = \,\,6\,\,\) મિનિટ