\(\frac{{{R_1}}}{{{t_1}}} = \frac{{{R_2}}}{{{t_2}}}\,\, \Rightarrow \,\,\,\,\frac{{{R_1}}}{{{R_2}}} = \frac{{{t_1}}}{{{t_2}}}\,\,\,\,\,........\,\,(i)\)

પરંતુ  \(\,R \propto {l}\,\,\, \Rightarrow \,\,\,\frac{{{R_1}}}{{{R_2}}} = \frac{{{{l}_1}}}{{{{l}_2}}}\,\,\,........\,\,(ii)\)

\({\text{(1)}} \) અને \({\text{(2)}}\) પરથી \(\frac{{{{l}_1}}}{{{{l}_2}}} = \frac{{{t_1}}}{{{t_2}}}\,\,\,.......\,\,(iii)\)

\({{l}_2} = \frac{2}{3}\,\,{{l}_1}\,\,\,\)  

\( \Rightarrow \,\,\frac{{{{l}_1}}}{{{{l}_2}}} = \frac{3}{2}\)

\(⇒\)પરિણામ \({\text{(3)}}\) પરથી  \(\,\frac{3}{2} = \frac{{15}}{{{t_2}}}\,\, \Rightarrow \,\,{t_2} = 10\,\,\,{\text{minutes }}\)