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Bolts installed on a production line are tightened with automatic wrenches. They are to be tightened sufficiently to yield the full cross-section in order to produce the highest possible initial tension. The limiting condition is twisting off the bolt head during assembly. The bolts have a mean twisting off torque of 20 Nm with a standard deviation of 1 Nm. The automatic wrenches have a standard deviation of 1.5Nm. What mean value of torque wrench setting would result in only 1 in 500 twisting off during assembly? Give a rough estimate only.