Prediction on paper

Take a piece of paper and write on it "seven" and hide it from the audience. Place two stacks of cards on the table and ask the viewer to choose one of the stacks. After that, the viewer discovers that your prediction has come true. The secret of the focus is that in one stack there are 4 sevens, and in the other pile 7 of any cards. Whichever stack the viewer chooses, and in one and in the other pile there is a number 7.