For a two-digit number, the ones digit is half of the tens digit. If these two digits are swapped, the new number obtained is 18 less than the original number. Find the original number. (column equation solution)
一个两位数，个位上的数字是十位上数字的一半，如果把这两个数字的位置交换，得到的新数比原数小18，求原数．(列方程解答)
<gadget id="calculator">21 - 10 - 2</gadget>
<output>9</output>

<gadget id="calculator">18 / 9</gadget>
<output>2</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>