The file Concrete 1 contains 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 distribution to perform this test?

c. Find the p-value in (a) and interpret its meaning.

Sample	Two Days	Seven Days
1	2.830	3.505
2	3.295	3.430
3	2.710	3.670
4	2.855	3.355
5	2.980	3.985
6	3.065	3.630
7	3.765	4.570
8	3.265	3.700
9	3.170	3.660
10	2.895	3.250
11	2.630	2.850
12	2.830	3.340
13	2.935	3.630
14	3.115	3.675
15	2.985	3.475
16	3.135	3.605
17	2.750	3.250
18	3.205	3.540
19	3.000	4.005
20	3.035	3.595
21	1.635	2.275
22	2.270	3.910
23	2.895	2.915
24	2.845	4.530
25	2.205	2.280
26	3.590	3.915
27	3.080	3.140
28	3.335	3.580
29	3.800	4.070
30	2.680	3.805
31	3.760	4.130
32	3.605	3.720
33	2.005	2.690
34	2.495	3.230
35	3.205	3.590
36	2.060	2.945
37	3.425	4.030
38	3.315	3.685
39	3.825	4.175
40	3.160	3.430