Matilda wants nothing more than for her parents to be good, smart people who love her. They are, of course, none of these things, so Matilda’s revenge game is the only thing keeping her sane. And being a small five-year-old girl, the only thing Matilda has to make her more powerful than her family members are her brains. In every other way, she has to do everything they tell her to do.
Keep in mind that Matilda’s pranks don’t have a lasting effect on her parents. They force her parents to behave for a bit, but then they revert to their old ways. While the novel insists that this is sad, it still suggests that the pranks serve an important purpose: to keep her sane, and to keep her working toward getting away from her parents.
Mr. Wormwood and Mrs. Wormwood are civil to Matilda for about a week after the parrot incident. But then, as Matilda and Michael are waiting in the living room for their TV dinners, Mr. Wormwood marches in looking very pleased with himself. He sits and tells Michael that he sold five cars today and is now rich. Then, he pulls out a piece of paper and says that since Michael is going to help him with the business one day, Michael must learn to calculate end-of-day profits.
The difference in the way Mr. Wormwood treats Matilda and Michael is startling: Michael is treated as the heir to the Wormwood fortune and business, while Matilda is ignored. Recall that Michael is, per the narrator, “normal,” so this just goes to show how misguided the Wormwoods are when it comes to how they treat their children.
After Michael fetches a pencil and paper, Mr. Wormwood reads off how much he paid for each car that he sold, and how much each car sold for today. Michael dutifully writes the numbers down and agrees that his father is brilliant. When he’s written down all the numbers, Mr. Wormwood tells Michael to figure out the profits. Michael is worried—“That’s a lot of sums”—but Mr. Wormwood says he has to learn. He notes that though he can’t do the math in his head, he worked it out on paper in under 10 minutes.
Mr. Wormwood isn’t entirely against education—he recognizes the need to be able to do simple addition, as that’s how he knows how successful his business has been. This may help explain why he dislikes Matilda so much: not only can she do far more than simple sums, Mr. Wormwood also has no intention of bringing her on as part of the business one day.
Quietly, Matilda says that Mr. Wormwood made 3,303.50 pounds. He scolds her to stop guessing, but then checks his final figure. Mr. Wormwood stiffens, turns red, and calls Matilda a cheat—she must’ve looked at his paper. Matilda points out that she’s across the room, but Mr. Wormwood shouts that no one, “especially a girl,” can do math like that. Just then, Mrs. Wormwood carries in everyone’s dinners: fish and chips she picked up on the way home from bingo. Mr. Wormwood tells his wife that Matilda is “a cheat and a liar” and turns on the TV.
Part of Mr. Wormwood’s dislike for Matilda, this passage shows, is rooted in sexism. Matilda is, to him, inferior to Michael because she’s female—and this makes her less intelligent and less worthy in her father’s eyes. This is, of course, a ridiculous stance; in addition to sexism being silly and baseless anyway, Matilda is clearly far more intelligent than her brother.