\(==> \)\(T \propto \frac{1}{{\sqrt {B\cos \phi } }} \)
\(\Rightarrow \frac{{{T_1}}}{{{T_2}}} = \sqrt {\frac{{{B_2}}}{{{B_1}}} \times \frac{{\cos {\phi _2}}}{{\cos {\phi _1}}}} \)
\(\Rightarrow \frac{{60/20}}{{60/15}} = \sqrt {\frac{{{B_2}}}{{{B_1}}} \times \frac{{\cos 60}}{{\cos 30}}} \)
\(\Rightarrow \frac{{{B_1}}}{{{B_2}}} = \frac{{16}}{{9\sqrt 3 }}.\)