Hence by using \(n' = n\left( {\frac{{v + {v_0}}}{v}} \right)\)
When passenger is sitting in train \(A\), then
\(5.5 = 5\left( {\frac{{v + {v_A}}}{v}} \right)\) …\((i)\)
when passenger is sitting in train \(B\), then
\(6 = 5\left( {\frac{{v + {v_B}}}{v}} \right)\) …\((ii)\)
On solving equation \((i)\) and \((ii)\) we get \(\frac{{{v_B}}}{{{v_A}}} = 2\)