What is the common ratio between 80, 72, 64.8?

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

The common ratio is 0.9 or the fraction 9/10

One that half its volume