You haven't given a specific question, but, if it helps, you can write a ratio as a fraction. a:b is the same as a/b. You can "cancel down" a ratio just the same as cancelling down a fraction, by dividing a and b by a common factor. Examples: 7:28 is the same as 1:4; 21:35 is the same as 3:5; 27:45 is also the same as 3:5. The fractions 7/28, 21/35 and 27/45 all cancel down in the same way. But ratios are not limited to two numbers: 2:6:10 is the same as 1:3:5.
For ratios consisting of just two numbers, don't forget you can convert them to fractions.
Another rule about ratios is, if you see a question like: the ratio of boys to girls in a class is 2:3 and there are 15 students in a class, how many girls are there? You add the ratio numbers together so 2+3=5, then divide the number of students by this number: 15/5=3. The ratio is telling you that 2/5 are boys and 3/5 are girls. 15/5 represents 1/5. To get the number of boys multiply 3 by 2 (2/5) and to get the number of girls multiply 3 by 3 (3/5). So that's 6 and 9, which of course add up to 15.
ANOTHER EXAMPLE
A farmer has sheep, cows and pigs in a certain part of his farm. The ratio of these in order is 16:13:7. If he has 180 animals in this area, how many of each type of animal does he have?
Add the ratios: 36. Divide this into the number of animals: 180/36=5. Now multiply the ratio numbers to get the numbers of each animal: 5*16=80 sheep; 5*13=65 cows; 5*7=35 pigs. Add these together=180 animals.