title: how to change the frame rate for a part of video using ffmpeg
body: <p>I want to change the frame rate of a video from 20 seconds to 30 seconds and want to keep the rest of the video running with the default frame rate using ffmpeg.</p>
  
  <p>is it possible to do the specified action using ffmpeg. if so please specify the command to do the same or else please help me to find the correct software in ubuntu to do the same.</p>
  
  <p>thanks in advance.</p>