7. If $t_{3}=20$ and $t_{20}=122$ then the common difference is A. 102 B. 6 C. 6.375 D. 17

Step 1 ：Given the 3rd term (\(t_3\)) and the 20th term (\(t_{20}\)) of an arithmetic sequence, we can write the arithmetic sequence formula: \(t_n = a + (n-1)d\)

Step 2 ：Using the given values, we have two equations: \(t_3 = a + 2d = 20\) and \(t_{20} = a + 19d = 122\)

Step 3 ：Solve the system of linear equations to find the value of d

Step 4 ：\boxed{d = 6}