The shape of a roof is an isosceles triangle, and a base angle of 30° is known, what is the number of its top angle.
一个房顶的形状是等腰三角形，已知一个底角30°，它的顶角度数是多少．
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">180 - 60</gadget>
<output>120</output>

<result>120</result>