Answer:
Find out the how much did each roll of crepe paper cost .
To prove
Let us assume that the cost of the roll of crepe paper be x.
Let us assume that the cost of the balloon be y .
As given
For party decorations, you buy 6 rolls of crepe paper and 20 balloons for $11.90.
Than the equation becomes
6x + 20y = 11.90
As given
After you start decorating, you determine that you need more party decorations.
You buy 2 more rolls of crepe paper and 15 more balloons for $6.05.
Than the equation becomes
2x + 15y = 6.05
Than two equations are
6x + 20y = 11.90 and 2x + 15y = 6.05
Multiply 2x + 15y = 6.05 by 3 and subracted with 6x + 20y = 11.90
6x - 6x + 45y -20y = 18.15 - 11.90
25y = 6.25
y = $0.25
Put in the equation 2x + 15y = 6.05
2x + 15 × 0.25 = 6.05
2x = 6.05 - 3.75
2x = 2.3
x = $1.15
Therefore the each roll of crepe paper cost is $1.15 .