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Algebra / Systems of two linear equations in two variables Difficulty: Hard

24 x + y = 48

6 x + y = 72

The solution to the given system of equations is (x,y). What is the value of y ?

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Explanation

The correct answer is 80 . Subtracting the second equation in the given system from the first equation yields 24x+y-6x+y=48-72, which is equivalent to 24x-6x+y-y=-24, or 18x=-24. Dividing each side of this equation by 3 yields 6x=-8. Substituting -8 for 6 x in the second equation yields -8+y=72. Adding 8 to both sides of this equation yields y=80

Alternate approach: Multiplying each side of the second equation in the given system by 4 yields 24x+4y=288. Subtracting the first equation in the given system from this equation yields 24x+4y-24x+y=288-48, which is equivalent to 24x-24x+4y-y=240, or 3y=240. Dividing each side of this equation by 3 yields y=80.