Cambridge University Entrance Interview Tricks | Can you Simplify ? ✍️🖋💙📘

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Order of magnitude suggests it starts with a 9. Modular arithmetic requires it to end with a 3. And the argument is a multiple of 3 (digit sum 54). So it's 9?3 where the missing digit is 3,6 or 9. Only 9 fits the order of magnitude. So 993.