A gas mixture contains $n_1$ moles of a monoatomic gas and $n_2$ moles of gas of rigid diatomic molecules. Each molecule in monoatomic and diatomic gas has 3 and 5 degrees of freedom respectively. If the adiabatic exponent $(C_P/C_V)$ for this gas mixture is $1.5$, then the ratio $n_1/n_2$ will be