Alex Bernadotte

let sum = 0

function add(val){
  if(val.checked){
     sum+= parseInt(val.value);
  } else {
    sum-= parseInt(val.value);
  }  
  document.getElementById('result')=sum;
}

function add(val){
  let sum = parseInt(document.getElementById('result'));
  if(val.checked){
     sum+= parseInt(val.value);
  } else {
    sum-= parseInt(val.value);
  }  
  document.getElementById('result')=sum;
}