Drop rate calculator

 <input type="number" id="num1" placeholder="First number" style="width:6em;">
 <select id="op">
   <option value="+">+</option>
   <option value="-">−</option>
   <option value="*">×</option>
   <option value="/">÷</option>
 </select>
 <input type="number" id="num2" placeholder="Second number" style="width:6em;">
 <button id="calcButton">=</button>
 

HTML test