Answer:
The quantity of frosted donuts = 2
The quantity of glazed donuts = 9
The quantity of custard filled donuts = 1
Step-by-step explanation:
Let the quantity of frosted donuts = 
Let the quantity of glazed donuts = 
Let the quantity of custard filled donuts = 
As per question statements, following equations can be made:

Putting
in (1) and (2):

Subtracting (4) from (5):

By equation (3):

By equation (1):

Therefore, the answers are:
The quantity of frosted donuts = 2
The quantity of glazed donuts = 9
The quantity of custard filled donuts = 1