I don't get these concepts for fractions

example
find each missing number
6 2/5 = 5 _?_/5

write each answer in the lowest terms subtract
12 1/3
- 9 2/3
________

Sometimes when we're adding or subtracting whole numbers and fractions, we need to change a whole number to a fraction. For the number

6 2/5, imagine 7 pizzas all divided into 5 pieces.

You ate 3 pieces of the 7th pizza so you have only 2/5 of it left.

Since the sixth pizza is divided into 5 pieces, you could say that you have five and 7/5 pizzas.

6 2/5 = 5 7/5

12 1/3 = 11 4/3

11 4/3
-9 2/3
_______
??