21 is not a prime number.
It is 17
Yes it is.
527/17 = 31 527 is not a prime number
No, it is not a prime number because it can be divided by 9 to get 3. A prime number is only divisible by itself and one.
The largest prime number less than twenty is 19. (BTW, I spotted a spelling error in the word twenty, but I assumed you meant twenty.)
No 23 is a prime #