6 the mean diameter holes produced by a drilling machine bit IS 4. 09 mm and the Standard deviation the diameters is 0 . 028 mm. For 20 holes drilled using this machine , determine, correct to the nearest Whole number , how many are likely to have diameters of between ( a ) 4 . 008 and 4 .021 mm and ( b ) 4.21 and 4.22 mm , assuming the diameters are normally distributed