Infernal Algebra

Earlier this evening, my friend and I were at the store. I picked a bottle of fine belgian ale out of the cooler. The bottle was priced at $4.59. When I got to the register, my friend was in line ahead of me buying a slice of pound cake. Knowing a good idea when I saw one, I went and picked a slice of pound cake also. I did not know the cost of the slice of cake. When the cashier rang up the slice of pound cake and the bottle of ale, the total was $6.66.

Today's date was Friday the 13th of October.

Q: What was the brand of the ale that I was buying?

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A. Duvel, of course.