Let D be the random variable denoting the diameter of this shop's bolts, so that D is normally distributed with ยต = 5.75 and ฯ = 0.07. The top 6% and bottom 6% of bolts have diameters dโ and dโ such that
P(dโ < D < dโ) = P(D < dโ) - P(D < dโ) = 0.94 - 0.06
i.e. dโ is the 94th percentile and dโ is the 6th percentile, for which
P(D < dโ) = 0.94
P(D < dโ) = 0.06
Convert D to a random variable Z following the standard normal distribution using
Z = (D - ยต) / ฯ
Then
P(D < dโ) = P((D - 5.75) / 0.07 < (dโ - 5.75) / 0.07)
0.94 = P(Z < (dโ - 5.75) / 0.07)
โ ย (dโ - 5.75) / 0.07 โย 1.55477
โ ย dโ โ 5.86
P(D < dโ) = P((D - 5.75) / 0.07 < (dโ - 5.75) / 0.07)
0.06 = P(Z < (dโ - 5.75) / 0.07)
โ ย (dโ - 5.75) / 0.07 โ -1.55477
โ ย dโ โ 5.64
So bolts with a diameter between 5.64 mm and 5.86 mm are acceptable.