Answer: A 6.12341234...
The digits 1234 repeat forever, so this repeating decimal is rational.
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Here's one way to show it is rational
x = 6.123412341234...
10000x = 61234.12341234...
10000x-x = 61234.12341234... - 6.123412341234...
9999x = 61228
x = 61228/9999
61228/9999 = 6.123412341234...