Let $x = the cost per flower.
Number of roses sold = 20
Profit on 20 roses = $6
Cost = (30 roses)($x per rose) = $ 20x
Revenue = ($0.50 per rose)*(20 roses) = $10
Profit = $10 - $ 20x = $6
That is,
10 - 20x = 6
20x = 10 - 6
20x = 4
x = $0.2 = 20 cents
Answer: She paid 20 cents per flower.