Vue

HTML

<div id="app">
  <h2>Todos:</h2>
  <ol>
    <li v-for="todo in todos">
      <label>
        <input type="checkbox" v-model="todo.checked">
        {{ todo.text }} <b v-if="todo.checked">( {{ todo.calc }} )</b>
      </label>
    </li>
  </ol>
  {{ calc }}
</div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

del {
  color: rgba(0, 0, 0, 0.3);
}

Vue

new Vue({
  el: "#app",
  data: {
    todos: [
      { text: "Learn JavaScript", val: 10, calc: 0 },
      { text: "Learn Vue", val: 20, calc: 0 },
      { text: "Play around in JSFiddle", val: 30, calc: 0 },
      { text: "Build something awesome", val: 40, calc: 0 }
    ]
  },
  computed: {
  	calc() {
    	var total = 0;
      
    	this.todos.forEach(function(item){
      	if(item.checked === true) {
        	item.calc = item.val * 2;
          total += item.calc;
        }
      });
      
      return total
		}
  }
})