Xiao Ming misunderstood the question when calculating addition. He mistook the 8 in the ones place as 3, and the 6 in the tens place as 9. The sum is 18.21. What should be the correct number?
小明计算加法时看错题，将个位上的8看成了3，十分位上的6看成了9，和是18.21，正确的得数应该是多少？
<gadget id="calculator">8 - 3</gadget>
<output>5</output>

<gadget id="calculator">0.9 - 0.6</gadget>
<output>0.3</output>

<gadget id="calculator">5 - 0.3</gadget>
<output>4.7</output>

<gadget id="calculator">18.21 + 4.7</gadget>
<output>22.91</output>

<result>22.91</result>