1.3, 2.4, 3.5, ___, 5.7, ... How many numbers should be filled in on the horizontal line
1.3，2.4，3.5，___，5.7，…横线上的数应填多少
<gadget id="calculator">3.5 + 1.1</gadget>
<output>4.6</output>

<result>4.6</result>