When is it necessaryto rename a mixed number when subtracting?

When doing sums with mixed numbers, it is often easier to convert the mixed numbers to improper fractions, do the sum and convert any resulting improper fraction back to a mixed number.

This is especially true of division, but with subtraction, if the fraction part of the second mixed number is larger than the fraction part of the first mixed number (subtracting the second from the first), this will result in requiring borrowing from the whole number of the first mixed number. Using improper fractions avoids this complication and makes all sums easier.