21 and 42 1

Logic Level 4

A set 'A' is defined to contain any positive integer which has the property of being a string of "21's", "42's", and/or "421's". If all the integers in set A are ordered from smallest to largest, what is the 21st term of this sequence?

For clarification, the sequence starts out: 21, 42, 421, 2121, etc.

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