DELETE mySQL query

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Is there a way to do the math in a DELETE statement (All 1 step; See my =
"sample" below pseudo code below). I am trying to delete the record if =
it is more than 3 hours old and the value of "approved" is still 0.

DELETE FROM `guest_book` WHERE `guest_book`.`date_submitted` + 3 hours =
=3D< '$current_time' AND `guest_book`.`approved` =3D '0' LIMIT 1
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