Brian begins by watching a 4-minute video, and since the second video is twice as long that means the second video is 4*2= <<4*2=8>>8 minutes.
The first two videos combined therefore for a 4+8=<<4+8=12>>12 minute run time.
Since the third video is twice as long as the first two videos put together, that means it is 12*2=<<12*2=24>>24 minutes in length.
The total length of time spent is therefore 24+12= <<24+12=36>>36 minutes
#### 36