How to reduce ratio to lowest term

Not easily:

First you have to create a method that finds all factors of a given number and output them as a IEnumerable<int>

public IEnumerable<int> Factorize(int number) {
    int max = (int)(number/2);  
    for(int factor = 1; factor <= max; ++factor) { 
        if(number % factor == 0) {
            yield return factor;
        }
    }
    yield number;
}
</int>

Now you have the factors, you need to find factors common to both numbers (common denominator)

public IEnumerable<int> CommonDenominators(int numberLeft,int NumberRight) {
    return Factorize(numberLeft).Union(Factorize(numberRight);
}
</int>

Actually:,You can adapt that to return the highest (i always thought is was the lowest, which would be 1?) common denominator:

public int HighestCommonDenominator(int numberLeft,int NumberRight) {
    return Factorize(numberLeft).Union(Factorize(numberRight).Max(n=>n);
}

Edit: If there are no other common denominators, i.e. 17:181 then the number 1 will be returned, so it's still accurate

Then just divide both numbers by that number:

int left = 550;
int right = 150:
int highestCommonDenominator = HighestCommonDenominator(left, right );
int lowestLeft = left / lowestCommonDenominator;
int lowestRight = right / lowestCommonDenominator;  

I haven't tested the code but the essentials are there

Hope that helps :)