Answer
1.4
Work Step by Step
We know that the specific heat ratio of the mixture is equal to the sum of the number of moles of each gas (each multiplied by their respective heat ratio) over the number of moles of gas. Thus:
$\gamma = \frac{(50)(1.29)+(30)(1.4)+(20)(1.67)}{50+30+20}=1.4$
