A tripod on a roof is an isosceles triangle. It is known that one of its base angles is 30 degrees, and what is its top angle in degrees.
一房顶的三角架为等腰三角形，已知它的一个底角为30度，它的顶角为多少度．
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

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

<result>120</result>