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