The cutting speed is reduced by 75%. Using the Taylor equation (VTn=C) for tool life, the percentage increase in tool life is (Take n = 0.5 and C = 400)
V2=V1−0.75V1=0.25V1
From Taylor equation,
V1Tn1=V2Tn2
⇒(T2T1)n=V10.25V1=4
⇒T2T1=(4)2=16
∴ % increase in tool life =(T2T1−1)×100=15×100=1500%