There are 180 people in the three groups. The sum of the number of people in the first and second groups is 20 more than that of the third group. The first group has 2 people less than the second group. Find the number of the first group.
三个小组共有180人，一、二两个小组人数之和比第三小组多20人，第一小组比第二小组少2人，求第一小组的人数．
<gadget id="calculator">51 - 2</gadget>
<output>49</output>

<result>49</result>