In an experiment involving the breaking strength of a certain type of thread used in personal flotation devices, one batch of thread was subjected to a heat treatment for 60 seconds and another batch was treated for 120 seconds. The breaking strengths (in N) of ten threads in each batch were measured. The results were

Find a 99% confidence interval for the difference in the mean strengths between threads treated for 60 seconds and those treated for 120 seconds.

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60 seconds: 43 52 52 58 49 52 41 52 56 54 120 seconds: 59 55 59 66 62 55 57 66 66 51