<script>
    function fun(x)
    {
        var b=document.getElementById('b'+x);
        b.value=(b.value*2);
    }
    function czysc()
    {
        for (i=0; i<10; i++)
        {
          var b=document.getElementById('b'+i);
          b.value=i;
        }
    }
</script>

<form>
  <input type="button" value="0" id="b0" onclick="fun(0)">
  <input type="button" value="1" id="b1" onclick="fun(1)">
  <input type="button" value="2" id="b2" onclick="fun(2)">
  <input type="button" value="3" id="b3" onclick="fun(3)">
  <input type="button" value="4" id="b4" onclick="fun(4)">
  <input type="button" value="5" id="b5" onclick="fun(5)">
  <input type="button" value="6" id="b6" onclick="fun(6)">
  <input type="button" value="7" id="b7" onclick="fun(7)">
  <input type="button" value="8" id="b8" onclick="fun(8)">
  <input type="button" value="9" id="b9" onclick="fun(9)"><br>
  <input type="button" value="czysc" onclick="czysc()">
</form>