Add a decimal point in the middle of a two-digit number to get a decimal. If the sum of this decimal and the original two-digit number is 86.9, what is the original two-digit number?
在一个两位数的中间加上小数点，得到一个小数，若这个小数与原来的两位数的和是86.9，则原来两位数是多少．
<gadget id="calculator">10 + 1</gadget>
<output>11</output>

<gadget id="calculator">86.9 / 11</gadget>
<output>7.9</output>

<gadget id="calculator">7.9 * 10</gadget>
<output>79</output>

<result>79</result>