Let the agular acceleratio be `alpha`. According to the question
`400 rev/min=0+alpha5s`………i
Let t be the time taken in attaining the speed of 200 rev/m whilch is half the maximum
Then, `200 rev/min=0+alphat` ………ii
Dividing i by ii we get,
`2=5s/t or t=2.5s`