Find the 20th term in the arithmetic sequence 15, 21, 27, ...

129

Step-by-step explanation:

The difference between 15 and 21 is 6, and that between 21 and 27 is also 6. Thus, the common difference is 6 and the first term is 15.

The rule for calculating the nth term of this sequence is therefore

a (n) = 15 + (n-1) 6

Check: Is the third term 27 when n = 3? Is a (3) = 15 + (3-1) 6 = 27? yes.

Thus, the 20th term is

a (20) = 15 + 19 (6) = 129