Answer:
$15, $20
Explanation:
Let u represent the unit price of umbrellas and h the unit price of hats.
Case 1: A. spent $80.
Then $80 = 4u + 5($4), or $80 = $20 + 4u, or $60 = 4u, or
u = $15/umbrella.
Case 2: A spent $100.
Then $100 = 4u + 5($4), or $100 = $20 + 4u, or $80 = 4u, or
u = $20/umbrella.
The least A. could have spent on a single umbrella was $15, and the most was $20.