Sample code:

Visualforce page:

<apex:page controller="Sample">

<apex:form >
   
    <apex:pageBlock >
        <apex:pageBlockSection >
            <apex:pageBlockSectionItem >
                <apex:outputLabel value="Value 1"/>
            </apex:pageBlockSectionItem>
            <apex:pageBlockSectionItem >
                <apex:inputText value="{!val1}"/>
            </apex:pageBlockSectionItem>                          
            <apex:pageBlockSectionItem >
                <apex:outputLabel value="Value 2"/>
            </apex:pageBlockSectionItem>
            <apex:pageBlockSectionItem >
                <apex:inputText value="{!val2}"/>
            </apex:pageBlockSectionItem>                         
            <apex:pageBlockSectionItem >
                <apex:selectRadio value="{!func}" layout="pageDirection">
                    <apex:selectOption itemValue="add" itemLabel="Add"/>
                    <apex:selectOption itemValue="sub" itemLabel="Subtract"/>
                    <apex:selectOption itemValue="div" itemLabel="Division"/>
                    <apex:selectOption itemValue="mod" itemLabel="Modulo Division"/>
                </apex:selectRadio>
            </apex:pageBlockSectionItem>
            <apex:pageBlockSectionItem >               
            </apex:pageBlockSectionItem>       
            <apex:pageBlockSectionItem >
                <apex:outputLabel value="Result"/>
            </apex:pageBlockSectionItem>
            <apex:pageBlockSectionItem >
                <apex:inputText value="{!result}" id="res"/><apex:actionStatus id="sts" startText="Finding..."/>
            </apex:pageBlockSectionItem>                                      
        </apex:pageBlockSection>   
        <apex:pageBlockButtons >
            <apex:commandButton value="Find" action="{!find}" reRender="res"  status="sts"/>
        </apex:pageBlockButtons>
    </apex:pageBlock>
   
</apex:form>

</apex:page> 

Apex Code:

public class Sample
{   
    public Double val1 {get;set;}
    public Double val2 {get;set;}
    public Double result {get;set;}
    public String func {get;set;}
   
    public Sample()
    {
    }
   
    public void find()
    {
        if(func == 'add')
        {
            result = val1 + val2;
        }
        else if(func == 'sub')
        {
             result = val1 - val2;
        }
        else if(func == 'div')
        {
             result = val1 / val2;
        }
        else
        {
             Integer temp =  math.mod(Integer.valueOf(val1) , Integer.valueOf(val2));
             result = Double.valueOf(temp);
        }
    }
  
}

Output: