Answer:

Step-by-step explanation:
Given the system of two equations:

Multiply the first equation and the second equation by 60 to get rid of fractions:

Now multiply the first equation by 4 and the second equation by 5:

Subtract them:

Substitute it into the first equation:

The solution is
$1.26 per yard. See photo for explanation.
I think she bought 23 feet
You can not work it out because we do not know the shape or the other properties of this poster.
Sorry for not being able to answer it.
I believe the answer is (a+c, b) but I'd double check to make sure.