The answer would be -prandial because it means during or relating to dinner or lunch; during or relating to the eating of food. :)
When used as a verb it is: to allow, when used as a noun it is: authorization
Think of an equation:
200 - 91 = 109
You can do anything to this equation, as long as you do the same thing to both sides. So, if you add 91 to both sides, you will get:
200 = 200
Since this holds true, Susan's strategy to add her answer to 91 makes sense.
