There are four numbers, the sum of each of them and the average of the other three numbers is 19, 90, 20, 15 respectively, find the average of the original four numbers.
有四个数，其中的每一个数与另外三个数的平均数的和分别为19，90，20，15，求原来四个数的平均数．
<gadget id="calculator">19 + 90 + 20 + 15</gadget>
<output>144</output>

<gadget id="calculator">144 / 2 / 4</gadget>
<output>18</output>

<result>18</result>