A, B, C, and D have an average of 38 stamps, the average number of A and B is 42, and the average number of B, C, and D is 36. How many stamps does B have?
甲、乙、丙、丁四人平均有邮票38张，甲与乙的平均张数是42，乙、丙、丁三人平均张数是36，求乙有邮票多少张？
<gadget id="calculator">36 * 3</gadget>
<output>108</output>

<gadget id="calculator">42 * 2</gadget>
<output>84</output>

<gadget id="calculator">38 * 4</gadget>
<output>152</output>

<gadget id="calculator">84 - 152</gadget>
<output>-68</output>

<gadget id="calculator">108 + (-68)</gadget>
<output>40</output>

<result>40</result>