Answer:
21:14, 10:5, 12 to 4, 20/6, and 7:2
Step-by-step explanation:
first simplify all the ratios. 
the first one is already simplified 
12 to 4 = 3 to 1
20/6 = 10/3
21:14 = 3:2 
10:5 = 2:1
Next find the least common factor the the three digits, 1, 2, and 3 and which is 6. 
Multiply the simplified factors into the least common factor. So 7:2 will be multiplied by 6 so it will be 21:6
Do it with the rest of the factors and get
21:6, 18 to 6, 20/6, 9:6, and 12:6
Finally order them from least to greatest,
9:6, 12:6, 18 to 6, 20/6, and 21:6
or 
21:14, 10:5, 12 to 4, 20/6, and 7:2