The completed ratio is like the fraction 6/8 which reduces to 3/4. This means that 6 is 3/4 of 8. Or you can say 8 is 4/3 of 6. So the fraction can be used for the other incomplete ratios. We know that the first number in the ratio is smaller than the second and the fraction 3/4 applied to the second number gives us a smaller first number. So apply this to 10 and we get 3/4 of 10=3/4*10=3/2*5=15/2=7.5; and apply this fraction to 12=3/4*12=9. So the incomplete ratios are 7.5:10 and 9:12.