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Hi! Love your book so far. I just started reviewing…

Hi! Love your book so far. I just started reviewing. I looked at number 10 in the first section on page 29 and I was wondering how would you pick the correct answer if you chose to substitute 8 for d? Because both answer choices a and d work. Would you just try all the answer choices and choose different variable values and try again or is there a different way to approach it?

What am I not seeing?

4x + y = 7
2x – 7y = 1

If I multiply the second equation by 2, I can stack them and subtract:

4x + y = 7
4x – 14y = 2

So, 15y = 5, —> y = 3

Then: 2x – 7(3) = 1 –> 2x – 21 = 3 —> 2x = 24 –> x = 12
But: 4x + 3 = 7 –> 4x = 4 –> x = 1

What am I not seeing? The answer should be x= 5/3.