function calc(thisform){
  var ntall ;
    
  with(thisform) {
    if ( isNaN(tall.value)  || (tall.value.length == 0) ) {
      alert("正しい数値を入力してください！");
      return ;
    }
    
    ntall = eval(tall.value);
   
    kotae.value = Math.round((5/105)* ntall) ;
    king.value = eval(tall.value)-eval(kotae.value) ;
    tax.value =  Math.round((5/105)* ntall) ;
  }   
}