Let’s take the example a/4+b/11 where a and b are whole numbers. We can’t simply add the numerators and denominators. We have to find some common denominator to express the fractions. To do this we need a number that is divisible by both denominators. In this case it’s 4×11=44.
This is going to change a and b to 11a and 4b, that is, multiply a by 11 and b by 4. Now we have two fractions with the same denominator:
11a/44+4b/44. When we add these together we get 11a+4b 44ths.
So, if we had 3/4+5/11, the result would be (11×3+4×5)/44=(33+20)/44=53/44.