The ratio of the top angle to the bottom angle of an isosceles triangle is 1:4, what is the top angle of this triangle?
一个等腰三角形顶角与一个底角的比是1：4，这个三角形顶角是多少．
<gadget id="calculator">1 + 4 + 4</gadget>
<output>9</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">180 * (1/9)</gadget>
<output>20</output>

<result>20</result>