At Blackstone Cafe, a regular entree costs 18.00 while an entree off the children’s menu costs less. Cliff treats his niece to dinner at the cafe and spends 1/3 of a gift certificate on her children’s entree and a drink. Afterwards, she orders a $6.00 dessert and he pays for that as well. When Cliff has paid for all of his niece’s food, he has exactly enough money left on the gift certificate to pay for his regular entree. How much money was the gift certificate worth?