Among the 900 natural numbers from 100 to 999, if one number in each number is the average of the other two numbers, how many such three-digit numbers are there in total?
从100到999这900个自然数中，使得每一数中的一个数码是另外两个数码的平均数，那么这样的三位数共有多少．
<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">16 * 6</gadget>
<output>96</output>

<gadget id="calculator">16 + 96 + 9</gadget>
<output>121</output>

<result>121</result>