JSFiddle - React, Tailwind, and code Playground

HTML

<script>
function _click(){
        var theform = document.forms[0];
        var rv = 0;
        var sv = 0;

        for(var i=0;i<theform._radio.length;i++)
        {
                if(theform._radio[i].checked)
                {
                        rv = theform._radio[i].value;
                }
        } //radio


        for(var i=0;i<theform._select.options.length;i++)
        {
                if(theform._select.options[i].selected)
                {
                        sv = theform._select.options[i].value;
                }
        } //select


        var inpva = parseInt(document.getElementById("_input1").value);

        var aaaa= parseInt(sv*rv);

        if(!isNaN(inpva) && !isNaN(aaaa))
                document.getElementById("_input2").value= aaaa+inpva ;
        else
                document.getElementById("_input2").value= '' ;
}
</script>

<form>

<input type="radio" name="_radio" value="10">10</input>
<input type="radio" name="_radio" value="20">20</input>

<select name="_select">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>

<input type="text" name="_input1" id="_input1">
<button type="reset" value="">清除</button>
<button type="button" onclick="_click(this.form)">=</button>
<input type="text" readonly="true" name="_input2" id="_input2">


</form>