Divide 30 (30 people) by 12 (12 people), obtaining the multiplier 2.5. Now, mult. every measurement in the recipe by 2.5.
For example: Pretzels: 2.5 times 1.5 cups = 3.75 cups.
Find the new quantities of the other ingredients similarly (mult. each measurement by 2.5).
Once you've finished this, add together all the volumes of all the ingredients to obtain the number of cups of dry snack mix.