Each household in a residential building subscribes to 2 different newspapers. If the residents of this residential building only subscribe to three newspapers A, B, and C, among them, there are 30 copies of newspaper A, 34 copies of newspaper B, and 40 copies of newspaper C. So how many households subscribe to both newspaper B and newspaper C?
一栋居民楼里的住户每户都订了2份不同的报纸．如果该居民楼的住户只订了甲、乙、丙三种报纸，其中甲报30份，乙报34份，丙报40份．那么既订乙报又订丙报的有多少户．
<gadget id="calculator">30 + 34 + 40</gadget>
<output>104</output>

<gadget id="calculator">104 / 2</gadget>
<output>52</output>

<gadget id="calculator">52 - 30</gadget>
<output>22</output>

<result>22</result>