The disk's average angular speed during those 5 seconds was
(1/2) (100 + 300) rad/s = 200 rad/sec
Spinning for 5 sec at an average angular speed of 200 rad/s, the disk turned
(200 rad/s) x (5 s) = 1,000 radians
(That's 500/pi revolutions = roughly 159.2 revolutions.)