Question

What is the common ratio for 128,96,72,54

Answer 1

Argument
Let the common ratio be r
Let each term be numbered by t

r = t2/t1
r = 96 / 128 = 0.75

Try it again
r = t3/t2
r = 72 / 96
r = 0.75

Conclusion
the common ratio for the given series is 0.75

Remark. Any term can be calculated from the following formula.
tn = 128(0.75)^(n-1) So if you want the 4th term
tn = 128(0.75)^(4 -1)
tn = 128(0.75)^3
tn = 128 * 0.421875
tn = 54 which is exactly what it should be.