4/45-1/30; 17/18-11/12

4/45 - 1/30 = (4*2)/90 - (1*3)/90= 8/90-3/90= 5/90= 1/18
17/18 -11/12= (17*2)/36 -(11*3)/36 = 34/36 - 33/36 =1/36