A florist wishes to make bouquets of mixed spring flowers. Part of each bouquet is to be tulips at $5 a bunch and part is to be daffodils at $2 a bunch. How many bunches of each should she use to make 15 bouquets which she can sell for $3 a bouquet.

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