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by cramya » Sat Nov 29, 2008 9:54 pm
If we recongnize the expression given in the problem then we know its dealing wiht weighted average.

Rearrange the expression:

10x+20y/x+y

The average would be between 10 and 20 and hence 18 (since its given y>x therefore the average would be closer to 20 than 10. Hence we rule out 15 since its halfway between 10 and 20)

D)
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by logitech » Sat Nov 29, 2008 10:00 pm
10x+20y / (x+y) = k

10x+10y / (x+y) + 10y /(x+y) = k

10 + 10y /(x+y) = k

10 [ 1 + y/(x+y)] = k

So it all comes down to what y/(x+y) could be. We know that y>x

Min: y=x

y/2y = 0.5

Max: x=0

y/y = 1

So k needs to be between:

10(1+0.5) < K < 10 ( 1+1)

15 < K < 20

Only 18 will fit into this range.

Choose (D)
LGTCH
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