If the perimeter of an isosceles triangle is 35 cm and the ratio of the lengths of the two sides is 3:1, what is the length of the waist of the triangle in cm?
一个等腰三角形的周长是35厘米，其中两条边的长度比是3：1，这个三角形的腰长多少厘米．
<gadget id="calculator">3 + 3 + 1</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">35 * (3/7)</gadget>
<output>15</output>

<result>15</result>