Why is it more difficult to use repeated addition when multiplying two mixed numbers than it is to use repeated addition when you are multiplying a mixed number by a whole number?

The reason it is more difficult to use repeated addition when multiplying two mixed numbers when compared to multiplying a mixed number and a whole number is because when you are multiplying a mixed number by a whole number, you have a whole number of groups of the mixed number. Ex: 1 1/2 x 3 would be 3 groups of 1 1/2, so

1 1/2 + 1 1/2 + 1 1/2.

If this were 1 1/2 x 2 1/2, you have 2 whole groups of 1 1/2, but then you would need to represent 1/2 group which requires multiplying or dividing first.