Hello,

The concept used here,

V2/V1 = (d1 + t)/ (d2 + t), the velocity ratio

V1=Velocity of motor

V2=Velocity of shaft

d2=diameter of driven shaft

d1=diameter of driving shaft

V1 = 1500 rpm. V2 =750 rpm. d2= 160 mm

As t is not given we can neglect the thickness

750/1500=d1/160

d1=80mm

Hope this helped.