Question 21

If $$3x + 5y + 7z = 49$$ and $$9x + 8y + 21z = 126,$$ then what is the value of y?

$$3x + 5y + 7z = 49$$
multiplying with 3 on both sides
$$9x + 15y + 21z = 147$$
$$9x + 8y + 21z = 126$$
Subtracting we get
7y=21
y=3

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