Pick any term you want but the first one. Whichever term you pick, divide that over the previous term
If you picked the second term, then
(second term)/(first term) = 72/80 = 9/10 = 0.9
If you picked the third term, then
(third term)/(second term) = 64.8/72 = 0.9
The fact we get the same result confirms this sequence is geometric. Each new term is found by multiplying the last term by 0.9 or the fraction 9/10, which is the common ratio
Answer: The common ratio is 0.9 or the fraction 9/10