Cheryl says that the system of equations at the right MUST be solved by elimination rather than by substitution. Explain why Cheryl is not correct.
-7x + 12y = 13
7X-11y=-9

you can't solve x because it is 2 linear equation and you only have one equation

this is a direct variation proportion. y = kx

y is the total amount due, k is the constant of variation, and x is the number of texts sent.

plug in the 3.20 and the 16 to solve for the "k". this is the constant of proportionality. it is the value that will not change.

3.20 = k(16). solving you get $.20 per text.

the equation would then be y = .20x