Answer:
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
