When the whole class of 30 people line up, the 17th person from left to right is Xiao Liang, and the 21st person from right to left is Xiao Ming, so how many people are there between Xiao Ming and Xiao Liang?
全班30人排队时，从左到右第17人是小亮，从右到左第21人是小明，那么小明和小亮之间有多少个人．
<gadget id="calculator">30 - 17</gadget>
<output>13</output>

<gadget id="calculator">30 - 21</gadget>
<output>9</output>

<gadget id="calculator">30 - 13 - 9 - 2</gadget>
<output>6</output>

<result>6</result>