A10. The sizes of the interior angles of a certain triangle, measured in degrees, are $x-20^{\circ}, 2 x-40^{\circ}$, and $5 x$. What is $x$?
(A) $30^{\circ}$
(B) $40^{\circ}$
(C) $50^{\circ}$
(D) $100^{\circ}$
(E) $180^{\circ}$
A10 The sum of the interior angles of a triangle is $180^{\circ}$. From the equation

$$
\left(x-20^{\circ}\right)+\left(2 x-40^{\circ}\right)+5 x=180^{\circ}
$$

it follows that $x=30^{\circ}$.

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