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GMAT Data Sufficiency (DS) | If 10[m][square_root]3[/square_root][/m] and ADC is a right

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If AD =10\sqrt{3} and ADC is a right angle, what is the area of triangle ABC?

(1) AC = 20

(2)angle BAD=30^{\circ}

This was asked in the Kaplan free test, but I think that the answer is wrong.

How I think the answer could be solved:

[Reveal] Spoiler:
Since\triangle ABD is similar to \triangleADC
\frac{AB}{AC}=\frac{AD}{DC}=\frac{BD}{AD} --------------(a)

1) We're given that AC=20
So by Pythagoras Theorem, CD = 10
and from (a), we can substitute values of 10 and10\sqrt{3} in\frac{AD}{DC}=\frac{BD}{AD} ,

thus getting the value of BD

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