What is the missing number 20 0.8?

There are infinitely many possible answers. If the missing number is the second in the sequence, it could be part of an arithmetic progression and so equal 10.4, or it could be in geometric progression and so would be 4, or harmonic progression which would give 1/0.65 = 1.54, approx. Furthermore, he missing number cold be the first or third in the sequence.