title: Change size of train and test set from MNIST Dataset
body: <p>I'm using the MNIST and Keras for learning about CNNs. I'm downloading the MNIST database of handwritten digits under Keras API as show below. The dataset is already split in 60.000 images for training and 10.000 images for test (see <a href="https://keras.io/datasets/#mnist-database-of-handwritten-digits" rel="noreferrer">Dataset - Keras Documentation</a>).</p>
  
  <pre><code>from keras.datasets import mnist
  
  (x_train, y_train), (x_test, y_test) = mnist.load_data()
  </code></pre>
  
  <p>How can I join the training and test sets and then separate them into 70% for training and 30% for testing?</p>