What are prime numbers between 20?
"between" requires two numbers, for example, between 20 and 30 or between 20 and 1000. You have not specified the other number.
Are all negative rational numbers less than an improper fraction?
Yes, they are all less than, for example, 12/3
How do you describe O and K and A by binary number system?
In order to answer that, it would first be necessary to know
the numbers that "O", "K", and "A" represent.
What are the set of factors less than 6?
All of the numbers less than six can be factors, and all integers have a least one factor that is less than six. Here's a set of factors less than 6: (1, 2, 3, 4, 5)