The following are the Rockwell hardness numbers obtained for six aluminum die castings randomly selected from production lot A and eight from production lot B:

Use the U test at the 0.05 level of significance to test whether the castings of production lot B are on the aver-age equally hard or whether they are harder than those of production lot A.

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Production lot A: Production lot B: 75, 56, 63,70, 58, 74 63, 85. 77, 80,86, 76, 72, 82