A=4.04S=0.028A1=4.038A2=4.045B1=4.042B2= 4.048

6. The mean diameter of holes produced by a drilling machine bit is a mm and the standard deviation of the diameters is s 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) of and a2 mm, and (b) bi and b2 mm, assuming the diameters are normally distributed. 7 The average useful life of 25 atteries 1