Ashley,

If you listed the fractions with your question, I am not seeing them. So I will give a generic answer to this question. Assuming that the numbers you provided which are 9, 11, 4 and 5 are all denominators of fractions, their common denominator will be 9*4*5*11 because none of those numbers have any factors that are shared

The factors for 9 are 3 and 3, the factors for 4 are 2 and 2 and the other two numbers 11 and 5 are prime numbers and therefore have no factors. 9 and 4, the only numbers with factors do not have shared factors, the common denominator is the product of multiplying each denominator

To demonstrate a scenario with denominators with common factors, let's say we need a common denominator for 24 and 36. They both have 12 as a common factor. 12 "goes into" 24 twice (12*2=24) and 36 is 12*3. So in this case the common denominator will be 12*2*3=72

Having said that, there is no harm in 24*36 as the common denomintor, but we use the above method for keeping the numbers small and making calculations easy