1. The "Pioneer" spacecraft travels back and forth between "Earth" and the "M78 Nebula". If the speed from "Earth" to "M78 Nebula" is 30000 kilometers/hour, and the speed from "M78 Nebula" to "Earth" is 20000 kilometers/hour, then the average speed of the "Pioneer" spacecraft for the round trip is $\qquad$ kilometers/hour.
$24000$