Answers:
19 Supergirl key chains
8 Wonder Woman key chains
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Explanation:
x = number of Supergirl key chains
y = number of Wonder Woman key chains
Francesca bought 27 total, so the two subtotals x and y must add to the grand total 27. The first equation is therefore x+y = 27. We can isolate y to get y = 27-x after subtracting x from both sides. We will use this equation in the step labeled "substitution step", shown below.
The second equation is 3x+2y = 73. The 3x refers to the total amount of money spent on just the Supergirl key chains ($3 a piece, x number of them, 3*x = 3x). Similarly, the 2y refers to the amount of money spent on just the Wonder Woman key chains, as she bought y of them for $2 a piece. In total, the final bill adds to 3x+2y which we are told is $73. That's how we end up with 3x+2y = 73
We will use the substitution property to get rid of the y variable, allowing us to solve for x
3x+2y = 73
3x + 2( y ) = 73
3x + 2(27 - x) = 73 .... substitution step: replace y with 27-x
3x + 2(27) + 2(-x) = 73 .... distribute
3x + 54 - 2x = 73
3x - 2x + 54 = 73
x + 54 = 73
x+54 - 54 = 73 - 54 ... subtract 54 from both sides
x = 19
Francesca bought 19 Supergirl key chains
Use this x value to find y. Go back to y = 27-x
y = 27 - x
y = 27 - 19 ... replace x with 19
y = 8
She also bought 8 Wonder Woman key chains
So the ordered pair solution is (x,y) = (19,8). If you were to graph the two equations x+y = 27 and 3x+2y = 73, then the two lines would cross at the location (19,8)
