Which of the following pairs are coprime ?

Dear student, 
(i) 48=7x7
   81=9x9
 The co prime pair is (i) 49 and 81.Because they have 1 as their highest common factor.

(ii) 18=2x3x3
      30=2x3x5
Since 2 amd 3 are common factor so they're not co-prime