We are given
selling price of one flower is $0.50
so, 
Let's assume
she sold 'n' flowers to get $20
so, total amount she get after selling 'n' flowers is


This is supposed to be $20
so, we can set it equal to 20

now, we can solve for n
we get

we are given
she makes $6.00 on every $20 worth sold
so, we can find cost of 40 flowers is 20-6
total cost is 14
or cost of 40 flowers is $14
now, we can find cost of one flower


so, cost of each flower is 35 cents...............Answer