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Hi,
Find the critical value tc for the confidence level c=0.99 and sample size n=26
Note: Degrees of Freedom = (n-1) = 25, You may find Below table useful in Your studies
DOF	Probability, p			
	0.1	0.05	0.01	0.001
1	6.31	12.71	63.66	636.62
2	2.92	4.3	9.93	31.6
3	2.35	3.18	5.84	12.92
4	2.13	2.78	4.6	8.61
5	2.02	2.57	4.03	6.87
6	1.94	2.45	3.71	5.96
7	1.89	2.37	3.5	5.41
8	1.86	2.31	3.36	5.04
9	1.83	2.26	3.25	4.78
10	1.81	2.23	3.17	4.59
11	1.8	2.2	3.11	4.44
12	1.78	2.18	3.06	4.32
13	1.77	2.16	3.01	4.22
14	1.76	2.14	2.98	4.14
15	1.75	2.13	2.95	4.07
16	1.75	2.12	2.92	4.02
17	1.74	2.11	2.9	3.97
18	1.73	2.1	2.88	3.92
19	1.73	2.09	2.86	3.88
20	1.72	2.09	2.85	3.85
21	1.72	2.08	2.83	3.82
22	1.72	2.07	2.82	3.79
23	1.71	2.07	2.82	3.77
24	1.71	2.06	2.8	3.75
	1.71         2.06        2.79	 3.73
26	1.71	2.06	2.78	3.71
27	1.7	2.05	2.77	3.69
28	1.7	2.05	2.76	3.67
29	1.7	2.05	2.76	3.66
30	1.7	2.04	2.75	3.65