Question

When you subtract mixed numbers, how do you know when you need to regroup?

Answer 1

do your own work...jk

Answer 2

It will be easier to explain if give some example or problem.
e.g. 3 1/4 – 2 1/2
= (3 + 1/4) – (2 + 1/2)
= 3 – 2 + 1/4 – 1/2
= 1 – 1/4
= 3/4