Astrip of paper is 9/10 of an inch long. you need to cut the paper into 3/12 inch pieces. will you be able to cut it?

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.