There are four numbers, the average of these four numbers is 21, the average of the first two numbers is 15, and the average of the last three numbers is 26, what is the second number?
有四个数，这四个数的平均数是21，其中前2个数的平均数是15，后3个数的平均数是26，第二个数是多少？
<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">26 * 3</gadget>
<output>78</output>

<gadget id="calculator">30 + 78</gadget>
<output>108</output>

<gadget id="calculator">21 * 4</gadget>
<output>84</output>

<gadget id="calculator">108 - 84</gadget>
<output>24</output>

<result>24</result>