Use a rope to tie the two wine bottles as shown in the figure 4 times. The outer diameter of the wine bottle is 6 cm, and a 15 cm rope is needed for the knot. How long is this rope?
用绳子将如图两个酒瓶捆4圈，酒瓶的外直径是6厘米，打结处需要15厘米的绳子，问这根绳子大约多长？
<gadget id="calculator">18.84 * 4</gadget>
<output>75.36</output>

<gadget id="calculator">75.36 + 15</gadget>
<output>90.36</output>

<result>90.36</result>