The amplitude of upper and lower side bands of A.M. wave where a carrier signal with frequency 11.21 MHz , peak voltage 15 V is amplitude modulated by a 7.7 kHz sine wave of 5 V amplitude are $\frac{\mathrm{a}}{10} \mathrm{~V}$ and $\frac{\mathrm{b}}{10} \mathrm{~V}$ respectively. Then the value of $\frac{a}{b}$ is $\_\_\_\_$ .