20000
19910
20100
19990

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Each of the thousand possible three-digit prefixes has ten four-digit extensions that use it as the first three digits, and another ten that use it as the last three digits. 1000 x 20 = 20000. The only numbers this overcounts are those made up entirely of a single digit, like 333-3333, of which there are 10. 20000 - 10 = 19990.