84/100 * 83/99 * 82/98 * 81/97 * 80/96 * 79/95 * 78/94 * 77/93 * 76/92 * 75/91 * 74/90 * 73/89 * 72/88 * 71/87 * 70/86 * 69/85 * 68/84 * 67/83 * 16/82

I don't want to multiply that out, but the first 17 fractions represent the cumulative odds of drawing a non deja vu card from the deck, which starts out with 84 non deja vu cards and 100 cards total; both obviously drop by 1 each time a card is drawn. The last fraction, 16/82, represents the odds of drawing the first one of the 16 deja vus as the 18th card.