Let
x = number of tulips
y = amount of daffodils
We have the following system of equations:
x + y = 15
5x + 2y = 45
Solving the system
Step 1:
-5x + -5y = -75
5x + 2y = 45
Step 2:
-3y = -30
y = 30/3 = 10
Step 3:
x = 15-y
x = 15-10
x = 5
Answer:
she should use
10 daffodils
5 tulips