What number should replace the X to the magic square?

An elderly couple was watching television one evening. The wife said, "I am going to get a dish of ice cream now." Kindly, the husband offered to get the ice cream for his wife. "I'll write it down so you don't forget," she said.
"I won't forget," the old gent said. "But, I want chocolate syrup and nuts on it. So, I'll write it down," she replied.
"I will get you the ice cream. Don't you worry," replied the gentleman.
A few minutes later, the old man returned with bacon and eggs. His wife said, "See, I should have written it down because you forgot the toast."