daisiesThere are 3 types of flowers.
Let C denote carnations, R denotes roses and D denote daisies.
Billy's Restaurant ordered 200 flowers for Mother's Day.
Mathematically,

They ordered carnations of $1.50 each, roses at $5.75 each, and daisies of $2.60 each.
The total order came to $589.50.
Mathematically,

They ordered mostly carnations, and 20 fewer roses than daisies.
Mathematically,

So, we have 3 equations and 3 unknowns.
Let us substitute eq. 3 into eq. 1

Let us substitute eq. 3 into eq. 2

So, now we have eq. 4 and eq. 5 with 2 unknowns. Let's solve them by substitution method.
Separate the variable C in eq. 4

Now substitute it into eq. 5.

So. we got the number of Daisies that is 70.
Now substitute D = 70 into the previous equation.

So. we got the number of Carnations that is 80.
Finally, from eq. 3 we get

So. we got the number of Roses that is 50.
Therefore.
Number of Carnations = 80
Number of Daisies = 70
Number of Roses = 50
Verification:
Total number of flowers should sum to 200.
80 + 70 + 50 = 200
200 = 200
Hence, we got the correct results.