There are 20 cards in red, yellow and green colors, among which 1 and 2 are written on the two sides of the red card, 1 and 3 are written on the two sides of the yellow card, and 2 and 2 are written on the two sides of the green card. 3. Now put the cards on the table so that the side with the larger number on each card is shown facing up. After calculation, the sum of the numbers displayed on each card is 56. If the front and back of all the cards are turned over, the sum of the numbers displayed on each card is 31. May I ask: How many yellow cards are there?
有红、黄、绿三种颜色的卡片共20张，其中红色卡片的两面上分别写有1和2，黄色卡片的两面上分别写有1和3，绿色卡片的两面上分别写有2和3，现在把这些卡片放在桌子上，让每後卡片写有较大的数字的那面朝上显示出来．经计算，各卡片所显示的数字之和为56，如果把所有卡片的正反翻转一下，那么各卡片所显示的数字之和为31．请问：黄色卡片有多少张？
<gadget id="calculator">3 * 20</gadget>
<output>60</output>

<gadget id="calculator">60 - 56</gadget>
<output>4</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">20 * 1</gadget>
<output>20</output>

<gadget id="calculator">31 - 20</gadget>
<output>11</output>

<gadget id="calculator">16 - 11</gadget>
<output>5</output>

<result>5</result>