Count 'em

Today, just passing along, verbatim, one of Alex Bellos’ Guardian brainteasers from this week, if you haven’t seen it:

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“F is the first and the seventh letter of this sentence."

Using the sentence above as a model, fill in the gap in the following sentence to make it correct:

C is the first and the [….] letter of this sentence.

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What I like about this problem is not only that it’s easy-to-state and is recursive, but even though it sounds harder, it solves fairly quickly just working through it logically. However, in the end I’m not sure there’s any algorithmic or logical system for reaching the final answer other than brute-force trial-and-error with the ultimate solution candidates. Bellos simply states the answer with no further explication, and I give it below:

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Answer:

C is the first and the forty-sixth letter of this sentence.