There are 8 groups in a class, and two groups are responsible for one day's teaching and hygiene. If any two groups have cooperated, how many days will it take at least.
某班有8个小组，两个小组负责一天的教窒卫生，若任何两个小组都合作过，则至少需要多少天．
<gadget id="calculator">8 * 7</gadget>
<output>56</output>

<gadget id="calculator">2 * 1</gadget>
<output>2</output>

<gadget id="calculator">56 / 2</gadget>
<output>28</output>

<result>28</result>