Question

A sequence has a third term of 18, and a common ratio of 3.

What is the fifth term?

6
54
162
486

Answer 1

$$
a_n = a\,r^{n-1}.
$$
where r is your common ratio and n denotes the term number, 1,2,3, etc

$$
a_5=6\times3^{5-1}=?
$$

Answer 2

$$
a_0=6\times3^{-1}=6/3=2\\
a_1=6\\
a_2=6\times3=18\\
a_3=6\times3^2=54\\
a_4=6\times3^3=162\\
$$
These are called geometric series:
http://en.wikipedia.org/wiki/Geometric_progression

Answer 3

a, b, c
given c = 18 ratio = 3
c/b =3

Answer 4

18 x 3 = 54
54 x 3 = 162