You need to find a number that can go into both 3 and 12. 3 can go into 3 and 12. So, you would then find out, how many times does 3 go into this number? Well, 3 goes into 3 once of course, and 3 goes into 12 four times. (3 x 4 = 12) In a fraction, this would be 1/4. (1/4 is the simplification of 3/12)
