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.form {
  display: flex;
}

.field {
  flex: 1 1 75%;
}

.btn {
  flex: 1 1 25%;
}

@media all and (max-width: 768px) {
  .field, .btn {
    flex-basis: 50%;
  }
}

@media all and (max-width: 480px) {
  .form {
    flex-wrap: wrap;
  }

  .field, .btn {
    flex-basis: 100%;
  }
}

<form class="form" action="#">
  <input class="field" value="text"/>
  <button class="btn">OK</button>
</form>