Cool Card Tricks Revealed - TO TELL THE NUMBERS ON TWO UNSEEN CARDS

As in the preceding trick, the ace counts eleven, and the court cards ten each. Let the person who chooses the two cards lay them on the table with their faces downward, and place on each as many as will make their number twenty-five.

Take the remaining cards and count them, when they will be found to be just as many as the points in the two cards.
For example, take an ace and a queen, i. e. eleven and ten, and lay them on the table. On the ace you must put fourteen cards, and on the queen fifteen.
There will be then fifteen cards in one heap and sixteen in the other: these added together make thirty-one cards: these subtracted from the number of cards in the pack, i. e. fifty-two, leave twenty-one, the joint number of the ace and the queen.

Excerpt from the book:
EVERY BOY’S BOOK: A COMPLETE ENCYCLOPÆDIA OF SPORTS AND AMUSEMENTS.
EDITED BY EDMUND ROUTLEDGE.
With more than Six Hundred Illustrations
FROM ORIGINAL DESIGNS.
LONDON: GEORGE ROUTLEDGE AND SONS,
THE BROADWAY, LUDGATE.
NEW YORK: 416, BROOME STREET.
1869.