The data in represent the compressive strength, in thousands of pounds per square inch (psi), of 40

Question:

The data in represent the compressive strength, in thousands of pounds per square inch (psi), of 40 samples of concrete taken two and seven days after pouring.
a. At the 0.01 level of significance, is there evidence that the mean strength is lower at two days than at seven days?
b. What assumption is necessary about the population in order to perform this test?
c. Find the p-value in (a) and interpret its meaning. Distribution
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