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Loved it. This would have been up there with Oregon Trail back in middle school in the computer lab (I'm dating myself a little). I was impressed with how it ramped up as well. Just as I was about to think "this is too repetitive" you throw in other operators and order of operations. My only suggestion is maybe have some sort of life/health that is lost when you get it wrong. To discourage brute forcing it until you get it right. If you were to take it further it would be interesting to have an attack mechanic to pickup the numbers. Lend itself to the dungeon/castle theme. You could then expand to other subjects, like spelling. The level would have a riddle or sentence with a blank and you'd have to solve it by spelling the word out of a bunch of letters. Great job.