Please note that if you mult. 243 by 2/3, you get 162.
If you mult. 162 by 2/3, you get 108.
If you mult. 108 by 2/3, you get 72.
We can then safely conclude that the common ratio is 2/3.
Our last given set member was 72.
Mult. this by 2/3 to get the 5th member: 48.
Mult. this 48 by 2/3 to get the 6th member: 32.
Mult. this 32 by 2/3 to get the 7th and last desired member: 21 1/3
