4. Before and after tests are to be made on the breaking strengths (oz) of a certain type of yarn. Specifically, seven determinations of the breaking strength are made on test pieces of the yarn before a spinning machine is reset and five on test pieces after the machine is reset, with the following results:

Before: 22.7 25.7 20.7 26.7 21.2 19.2 22.7 After: 23.2 23.7 25.2 23.7 24.7 Assuming that determinations made “after reset” and determinations made “before reset” are independently distributed as N(μ1, σ2 1) and N(μ2, σ2 2), respectively, find a 95% confidence interval for μ1 − μ2 without assuming equal variances.