Get the two numbers into the same denominator to compare them.
The LCD of 10 and 12 is 60, but you could use 120 or 240 or whatever else.
9/10 would scale up to 54/60 (10 times 6 = 60, 9 times 6 = 54)
3/12 would scale up to 15/60 (12 times 5 = 60, 3 times 5 = 15)
Comparing 15/60 to 54/60, you could cut 3 pieces and would have 9/60 of an inch (3/20) remaining.
