The next number in the series would be 81.

The pattern in the series is that the tenths place is multiplied by two, while the ones place is divided by two.

Starting with 18, multiplying 10 by 2 gives 20 and dividing 8 by 2 gives 4 which is 24, Multiplying 20 by two and dividing 4 by 2 yields 42 and finally multiplying 40 by two and dividing 2 by 2 gives 81.
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