How do know when to borrow or rename a mixed number

Question

like this one 15 3/8 - 11 1/5. Write the answer in simplest form

Answer 1

Change the fractions to the same denominator:  15/40 and 8/40.  You don't need to borrow with this one because 15-8 is 7, so the fraction is 7/40 and the whole number is 4.  So 4 7/40