First we convert it in seconds.
Number of seconds in 1 minute = 60 seconds
Therefore,
Number of seconds in 4.2 minutes = 4.2 x 60 = 252 seconds.
Next we convert it in hours.
60 minutes make up an hour. So we can write,
Number of hours in 60 minutes = 1 hour
Number of hours in 1 minute = 1/60 hour
Number of hours in 4.2 minutes = 1/60 x 4.2 = 0.07 hour
Therefore, the video is 252 seconds or 0.07 hour long.