text: function tabuada() {
      let num = document.getElementById('txtn')
      let tab = document.getElementById('seltab')
      if (num.value.length == 0) {
          window.alert('Por Favor digite um Numero!')
      } else {
          let n = Number(num.value)
          let c = 1
          tab.innerHTML = ''
          while (c <= 10) {
              let item = document.createElement('option')
              item.text = `${n} x ${c}= ${n*c}`
              item.value = `tab${c}`
              tab.appendChild(item)
              c++
          }
      }
  }