A standard air-filled rectangular waveguide with dimensions \( a = 8 \, \text{cm}, b = 4 \, \text{cm} \), operates at 3.4 GHz. For the dominant mode of wave propagation, the phase velocity of the signal is \( v_p \). The value (rounded off to two decimal places) of \( v_p / c \), where \( c \) denotes the velocity of light, is