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Question

If 2x(x + y + z) = 250, 2y(x + y + z) = 100, 2z(x + y + z) = 100, then find the value of (3x + 6y + 15z).

This question was previously asked in
RRB NTPC 2015 CBT 1 Question Paper PDF (2-Apr-2016) (Shift 3)
The correct answer is

95

Add the three given equations: $2x(x+y+z) + 2y(x+y+z) + 2z(x+y+z) = 250 + 100 + 100$, so $2(x+y+z)^2 = 450$, giving $(x+y+z)^2 = 225$ and $x+y+z = 15$.

Substitute $x+y+z = 15$ back: $2x \times 15 = 250 \Rightarrow x = 25/3$; $2y \times 15 = 100 \Rightarrow y = 10/3$; similarly $z = 10/3$.

Then $3x + 6y + 15z = 3(25/3) + 6(10/3) + 15(10/3) = 25 + 20 + 50 = 95$.

Hence, 95.

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