Answer: $\large\frac{\gamma -1}{\gamma}$

For an isobaric process, $Q = nC_P \Delta T$ and $W = nR \Delta T$

$\Rightarrow \large\frac{\Delta W}{\Delta Q}$$ = \large\frac{R}{C_P}$

Now, $C_P = \large\frac{\gamma R}{ \gamma -1}$ for an isobaric process, where $\gamma$ is the adiabatic index or the heat capacity ratio.

$\Rightarrow \large\frac{\Delta W}{\Delta Q}$$ = \large\frac{R}{C_P}$$ =\large\frac{\gamma -1}{\gamma}$