No.
Yes, 90 and 91 are relatively prime. Any consecutive integers will be relatively prime.
No, they are not relatively prime.
It can be. 21 is relatively prime with 32.
153
Yes.
Yes.
Yes.
Yes.
The numbers 15, 32, and 49 are relatively prime because their greatest common factor is 1.
90 = 2*32*5
90 = 21 x 32 x 51