Mario sees popcorn and pretzels as perfect substitutes. he is always willing to substitute 3 cups of popcorn for 1 cup of pretzels. if the price of a cup of popcorn equals the price of a cup of pretzels, mario will eat:

If the price of a cup of popcorn is equals with prize of cup of pretzels, Mario will likely buy and eat only the pretzels as the popcorn is already in equal with the pretzels in which he can now buy the pretzels without having to substitute it with the popcorn, making him eat and brought the pretzels.