9. When an earthquake occurs, both P-waves and S-waves are propagated from the focus. In a certain earthquake, the monitoring station received the P-waves first, and 20 seconds later, it received the S-waves. If the propagation speed of P-waves is 6 kilometers/second, and the propagation speed of S-waves is 4 kilometers/second, then the distance between the focus and the monitoring station is $\qquad$ kilometers.
$240$