The average of the three numbers is 15.7. After adding the fourth number, the average is 16. What is the fourth number?
三个数的平均数是15.7，加上第四个数后平均数是16，第四个数是多少
<gadget id="calculator">16 * 4</gadget>
<output>64</output>

<gadget id="calculator">15.7 * 3</gadget>
<output>47.1</output>

<gadget id="calculator">64 - 47.1</gadget>
<output>16.9</output>

<result>16.9</result>