Finding a common denominator between 10 and 12 to be 10*12=120, we can multiply 9/10 by 12/12 and 3/12 by 10/10 to get 108/120 and 30/120 respectively. Dividing the 2 (as we want to know how much 3/12 or 30/120 goes into 9/10 or 108/120), we get
(108/120)/(30/120)=(108/120)*(120/30)=(108/30)=3+18/30 (as 30 goes into 90 3 times and 108-90=18)